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Under Prompt Corrective Action Requirements, Ratio Banking Regulation, Total Risk-Based Capital Ratio, Well Capitalized, Minimum Subsequent Events Subsequent Events [Text Block] Other Property [Member] Other Property [Member] Private mortgage-backed securities Mortgage-Backed Securities, Issued by Private Enterprises [Member] Performing Financial Instruments [Member] Performing Financial Instruments [Member] Held-to-Maturity Estimated Fair Value, Within One Year Debt Securities, Held-to-Maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, tax expense (benefit) Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, tax expense (benefit) OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), Tax Amortization of unrealized gains (losses) on transfers of securities Amortization of unrealized gains (losses) on transfers of securities Amortization of unrealized gains (losses) on transfers of securities Statement [Line Items] Statement [Line Items] Total assets Assets Entity [Domain] Entity [Domain] Business Combination [Abstract] Business Combination [Abstract] Extended Maturity [Member] Extended Maturity [Member] Interest Expense, Operating Interest Expense, Operating Nonvoting, noncumulative preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Internal Credit Assessment [Domain] Internal Credit Assessment [Domain] Industry Sector [Axis] Industry Sector [Axis] Total interest income Interest and Dividend Income, Operating Financing Receivable, Credit Quality Indicator [Table] Financing Receivable, Credit Quality Indicator [Table] Commitments to purchase or sell Financing Receivable, Commitment to Purchase or Sell, Amount Net gains and write-downs of OREO and other assets pending disposal Net 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bearing Noninterest-Bearing Deposit Liabilities Increase (Decrease) in Deposits Increase (Decrease) in Deposits Interest on long-term debt Interest Expense, Other Long-Term Debt Measurement Basis [Axis] Measurement Basis [Axis] Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount Banking Regulation, Total Risk-Based Capital, Well Capitalized, Minimum Gross Amounts Offset in the Balance Sheet Derivative Asset, Subject to Master Netting Arrangement, Liability Offset Stock repurchased and retired (in shares) Stock Repurchased and Retired During Period, Shares Proceeds from sale of loans held-for-investment Proceeds from Sale, Loan, Held-for-Investment Goodwill Net carrying value at beginning of the period Net carrying value at end of period Goodwill Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) Fair Value Measurement Inputs and Valuation Techniques [Line Items] Total investment securities Investments (Increase) decrease in unrealized losses during the period Change in net unrealized gain during period, pre-tax OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax Actual, Ratio Banking Regulation, Tier 1 Risk-Based Capital Ratio, Actual Subsequent Event Type [Domain] Subsequent Event Type [Domain] Retained earnings Retained Earnings [Member] Retained Earnings [Member] Net unrealized loss on investment securities available-for-sale AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] Deferred loan fees and costs Financing Receivable, Unamortized Loan Cost (Fee) Fair Value Measurements Fair Value Measurement, Policy [Policy Text Block] OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax Fair Value Inputs, Assets, Quantitative Information Fair Value Measurement Inputs 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but Within Five Years Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost Proceeds from Sale, Maturity and Collection of Investments [Abstract] Proceeds from Sale, Maturity and Collection of Investments [Abstract] Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years Debt Securities, Held-to-Maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value Financing Receivable, Excluding Accrued Interest, Modified Period Financing Receivable, Excluding Accrued Interest, Modified in Period, Amount Provision for Loan, Lease, and Other Losses (Reduction of) provision for credit losses Provision for Loan, Lease, and Other Losses Off-Balance Sheet, Credit Loss, Liability Off-Balance-Sheet, Credit Loss, Liability Title and Position [Axis] Title and Position [Axis] Valuation allowance for loans held-for-sale Valuation allowance for loans 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Asset, Originating and Post Period [Domain] Financial Asset, Originating and Post Period Year Two, Originated, Fiscal Year Before Current Fiscal Year Year Two, Originated, Fiscal Year Before Current Fiscal Year [Member] Year Two, Originated, Fiscal Year Before Current Fiscal Year Cumulative Effect, Period of Adoption [Domain] Cumulative Effect, Period of Adoption [Domain] Collateral Type [Axis] Collateral Held [Axis] Commitments under construction contracts Unrecorded Unconditional Purchase Obligation Common dividends (in dollars per share) Common Stock, Dividends, Per Share, Declared Entity File Number Entity File Number Available-for-Sale Estimated Fair Value, Within One Year Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value Non-vested common shares issued Shares Issued, Value, Share-Based Payment Arrangement, after Forfeiture Maturities of Investment Securities Investments Classified by Contractual Maturity Date [Table Text Block] Proceeds from Life Insurance Policy Proceeds from Life Insurance Policy Financing Receivable, Excluding Accrued Interest, Revolving Financing Receivable, Excluding Accrued Interest, Revolving Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax Schedule of Allowance for Loan Losses by Portfolio Segment Financing Receivable, Allowance for Credit Loss [Table Text Block] Common Stock, Shares, Issued Common Stock, Shares, Issued Nonaccrual 30 or more DPD Nonaccrual 30 or more DPD [Member] Nonaccrual 30 or more DPD Entity Information [Line Items] Entity Information [Line Items] Basis of Presentation Business Description and Basis of Presentation [Text Block] Credit Derivatives Contract Type [Domain] Credit Derivatives Contract Type [Domain] Forward loan sales contracts Forward Contracts Forward Contracts [Member] Exempt from federal taxes Interest Income, Securities, Operating, Tax Exempt Banking Regulation, Common Equity Tier One Risk-Based Capital, Capital Adequacy, Minimum Banking Regulation, Common Equity Tier 1 Risk-Based Capital, Capital Adequacy, Minimum Gross Unrealized Losses, 12 Months or Longer Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Other Comprehensive Income/Loss Comprehensive Income (Loss) Note [Text Block] Equity Components [Axis] Equity Components [Axis] Held-to-Maturity Estimated Fair Value, After Ten Years Debt Securities, Held-to-Maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value Class of Stock [Line Items] Class of Stock [Line Items] Cumulative Effect, Period of Adoption [Axis] Cumulative Effect, Period of Adoption [Axis] Terminated Hedges [Axis] Terminated Hedges [Axis] Terminated Hedges Class A Common Stock Common Class A [Member] Entity Interactive Data Current Entity Interactive Data Current Noninterest expense: Noninterest Expense [Abstract] Net unrealized gain (loss) on derivatives Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Amount Banking Regulation, Tier 1 Leverage Capital, Well Capitalized, Minimum Entity Small Business Entity Small Business Comprehensive income, net of tax Comprehensive Income (Loss), Net of Tax, Attributable to Parent Financial Instruments Security Sold under Agreement to Repurchase, Subject to Master Netting Arrangement, Collateral, Right to Reclaim Security Not Offset Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio Banking Regulation, Tier 1 Leverage Capital Ratio, Well Capitalized, Minimum Subsequent Events [Abstract] Subsequent Events [Abstract] Fair Value, Recurring and Nonrecurring [Table] Fair 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Owned Real Estate and Real Estate Acquired in Settlement of Loans Proceeds from Sale of Wholly Owned Real Estate and Real Estate Acquired in Settlement of Loans Financing Receivable, Modifications to Borrowers [Table] Financing Receivable, Modifications to Borrowers [Table] Financing Receivable, Modifications to Borrowers [Table] Goodwill [Table] Goodwill [Table] Actual, Ratio Banking Regulation, Tier 1 Leverage Capital Ratio, Actual Share Repurchase Program, Remaining Authorized, Amount Share Repurchase Program, Remaining Authorized, Amount Subsequent Event [Line Items] Subsequent Event [Line Items] Minimum Required for Capital Adequacy Purposes, Amount Banking Regulation, Tier 1 Risk-Based Capital, Capital Adequacy, Minimum Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Fair Value of Financial Collateral in the Balance Sheet Security Sold under Agreement to Repurchase, Subject to Master Netting Arrangement, Collateral, Right to Reclaim Cash 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[Domain] Antidilutive Securities, Name [Domain] Accounting Standards Update and Change in Accounting Principle [Table] Accounting Standards Update and Change in Accounting Principle [Table] Finite-lived Intangible Assets [Roll Forward] Intangible Asset, Finite-Lived [Roll Forward] Increase (Decrease) in Accounts Payable and Accrued Liabilities Increase (Decrease) in Accounts Payable and Accrued Liabilities Weighted-Average Interest Rate Reduction Weighted-Average Interest Rate Reduction [Member] Weighted-Average Interest Rate Reduction Collateralized Mortgage Obligations Collateralized Mortgage Obligations [Member] Receivable Type [Axis] Receivable Type [Axis] Loan Restructuring Modification [Axis] Loan Restructuring Modification [Axis] Deferred tax benefit (expense) related to other comprehensive income Other Comprehensive Income (Loss), tax expense (benefit) Other Comprehensive Income (Loss), tax expense (benefit) Other Comprehensive Income (Loss), Tax Originated, More than Five 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government agencies US Government Agencies Debt Securities [Member] Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax Supplemental Cash Flow Information [Abstract] Supplemental Cash Flow Information [Abstract] Gain (Loss) on Sale of Loans and Leases Gain (Loss) on Sale of Loans and Leases Accounts payable and accrued expenses Accounts Payable and Accrued Liabilities Financial liabilities: Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] Short-Term Debt [Table] Short-Term Debt [Table] Income Tax Paid, before Refund Received Income Tax Paid, before Refund Received Proceeds from (Repayments of) Other Debt Proceeds from (Repayments of) Other Debt Year Five, Originated, Four Years before Current Fiscal Year Year Five, Originated, Four Years before Current Fiscal Year [Member] Year Five, Originated, Four Years before Current Fiscal Year Other assets Other Assets Total construction loans Real Estate Construction Financing Receivable [Member] Real Estate Construction Financing Receivable [Member] 30 to 59 Days Past Due Financial Asset, 30 to 59 Days Past Due [Member] Entity Tax Identification Number Entity Tax Identification Number Available-for-Sale, Amortized Cost Debt Securities, Available-for-Sale, Amortized Cost, Rolling Maturity [Abstract] Cash and due from banks Cash and Due from Banks Legal Entity [Axis] Legal Entity [Axis] Schedule of Recorded Investment in Past Due Loans by Class Financing Receivable, Past Due [Table Text Block] Financial Asset, Not Past Due Financial Asset, Not Past Due [Member] Unrealized loss on derivatives, net of tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, after Tax Assets acquired: Business Combination, Recognized Asset Acquired, Asset [Abstract] For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio Tier One 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Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax Other, including overdrafts Other Financing Receivable, Including Overdrafts [Member] Other Financing Receivable, Including Overdrafts [Member] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Derivative Contract [Domain] Derivative Contract [Domain] Hedging Relationship [Domain] Hedging Relationship [Domain] Payment to Acquire Life Insurance Policy, Investing Activities Purchases of company-owned life insurance Purchases of company-owned life insurance Total liabilities and stockholders’ equity Liabilities and Equity Liability Adjustments [Domain] Liability Adjustments [Domain] Liability Adjustments [Domain] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Industry Sector [Domain] Industry Sector [Domain] Common stock transactions: Common Stock Transactions [Abstract] Common Stock Transactions. Pass Pass [Member] Geographic Distribution [Domain] Geographic Distribution [Domain] Schedule of Loans by Class Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] FIB First Interstate Bank [Member] First Interstate Bank [Member] Subordinated debentures held by subsidiary trusts Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] Long-term debt Other Long-Term Debt Credit Derivatives Contract Type [Axis] Credit Derivatives Contract Type [Axis] Repossessed Assets [Abstract] Repossessed Assets [Abstract] Gain on sale of branches, net Gain on sale of branches, net Gain on sale of branches, net Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 5 through 10 Market Approach Valuation Technique Valuation, Market Approach [Member] Available-for-Sale Estimated Fair Value, After One Year but Within Five Years Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value Financing Receivable Portfolio Segment [Axis] Financing Receivable Portfolio Segment [Axis] Transfer of loans to other real estate owned Real Estate Owned, Transfer to Real Estate Owned Preferred Stock, Shares Authorized Nonvoting, noncumulative preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Loan Dependency [Domain] Loan Dependency [Domain] Loan Dependency Accruing Accruing 30 or more DPD [Member] Accruing 30 or more DPD Commercial Mortgage-Backed Securities Commercial Mortgage-Backed Securities [Member] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Class of Stock [Domain] Class of Stock [Domain] Interest Paid, Excluding Capitalized Interest, Operating Activity Interest Paid, Excluding Capitalized Interest, Operating Activity Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Hedging Designation [Axis] Hedging Designation [Axis] 60 to 89 Days Past Due Financial Asset, 60 to 89 Days Past Due [Member] Balance at beginning of year Real Estate Acquired Through Foreclosure [Roll Forward] Real Estate Acquired Through Foreclosure [Roll Forward] Total financial assets Assets, Fair Value Disclosure Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy and NAV [Domain] Schedule of Derivatives Instruments Statements of Financial Performance and Financial Position, Location Schedule of Derivatives Instruments Statements of Financial Performance and Financial Position, Location [Table Text Block] For Capital Adequacy Purposes Plus Capital Conservation Buffer, Ratio Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets Interest and fees on loans Interest and Fee Income, Financing Receivable Entity Filer Category Entity Filer Category Fair Value, Total Debt Securities, Available-for-Sale, Unrealized Loss Position Supplemental Cash Flow Elements [Abstract] Supplemental Cash Flow Elements [Abstract] Construction Contracts Construction Contracts [Member] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Credit Extension Commitments Credit Extension Commitments Credit Extension Commitments Payments for (Proceeds from) Other Loans and Leases Payments for (Proceeds from) Other Loans and Leases Amount of net fee income recognized in other noninterest income Fee Income on Derivatives Fee Income on Derivatives Available-for-Sale Amortized Cost, Within One Year Debt Securities, Available-for-Sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost Accrued interest receivable Interest Receivable Interest Receivable Payments to Acquire Investments [Abstract] Payments to Acquire Investments [Abstract] Liability Adjustments [Axis] Liability Adjustments [Axis] Liability Adjustments Financial Asset, Aging [Axis] Financial Asset, Aging [Axis] Not Designated as Hedging Instrument Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] Loans Held For Sale (Table) Loans Held For Sale (Table) [Table Text Block] Loans Held For Sale (Table) Investments, Debt and Equity Securities [Abstract] Investments, Debt and Equity Securities [Abstract] Depreciation, Depletion and Amortization Depreciation, Depletion and Amortization Deposits: Deposits [Abstract] For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in Gross Amounts Offset in the Balance Sheet Security Sold under Agreement to Repurchase, Subject to Master Netting Arrangement, Asset Offset Gross Amounts Recognized Security Sold under Agreement to Repurchase, Subject to Master Netting Arrangement, before Offset Stock, Class of Stock [Table] Stock, Class of Stock [Table] Right-of-Use Asset Obtained in Exchange for Finance Lease Liability Right-of-Use Asset Obtained in Exchange for Finance Lease Liability Life Insurance, Corporate or Bank Owned, Change in Value Life Insurance, Corporate or Bank Owned, Change in Value Net Amounts in the Balance Sheet Derivative liabilities Derivative Liability Leverage capital ratio: Banking Regulation, Tier 1 Leverage Capital [Abstract] Entity Address, Address Line One Entity Address, Address Line One Finance Lease, Right-of-Use Asset, Amortization Right-of-Use Asset Obtained in Exchange for Operating Lease Liability For Capital Adequacy Purposes Plus Capital Conservation Buffer, Amount Capital Required For Capital Adequacy Under Basel III Fully Phased-In Capital Required for Capital Adequacy under Basel III Fully Phased-in Provision for income tax Income Tax Expense (Benefit) Noninterest Expense Noninterest Expense Amount of net gains recognized in mortgage banking revenues Gain (Loss) on Fair Value Hedges Recognized in Earnings Collateralized Loan Obligations Collateralized Loan Obligations [Member] Stock Issued During Period, Shares, Restricted Stock Award, Forfeited Non-vested common shares forfeited (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Forfeited Combination Term Extension and Interest Rate Reduction Percent Combination Term Extension and Interest Rate Reduction Percent Combination Term Extension and Interest Rate Reduction Percent Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect, Total Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Schedule of Impaired Financing Receivable [Table] Schedule of Impaired Financing Receivable [Table] Derivative Liabilities, Notional Amount Derivative Liability, Notional Amount Derivative Liability, Notional Amount Recent Authoritative Accounting Guidance Accounting Standards Update and Change in Accounting Principle [Text Block] Net income Net income Net income Net Income (Loss) Attributable to Parent Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Loan Restructuring Modification [Domain] Loan Restructuring Modification [Domain] Statement [Table] Statement [Table] Financing Receivable, Past Due [Table] Financing Receivable, Past Due [Table] Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements, Ratio Banking Regulation, Tier 1 Risk-Based Capital Ratio, Well Capitalized, Minimum Derivative Liabilities, Estimated Fair Value Interest Rate Derivative Liabilities, at Fair Value Schedule of Accumulated Other Comprehensive Income (Loss) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Interest on deposits in banks Interest Income, Domestic Deposits Financing Receivable, Excluding Accrued Interest, Year Two, Originated, Fiscal Year before Current Fiscal Year Financing Receivable, Excluding Accrued Interest, Year Two, Originated, Fiscal Year before Current Fiscal Year Non-interest income Other Service Charges, Commissions, and Fees Other Service Charges, Commissions, and Fees Liabilities assumed: Business Combination, Recognized Liability Assumed, Liability [Abstract] OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after Tax OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after Tax Gross Amounts Recognized Derivative Asset, Subject to Master Netting Arrangement, before Offset Recoveries Collected Financing Receivable, Allowance for Credit Loss, Recovery Geographic Distribution [Axis] Geographic Distribution [Axis] Business Assets [Member] Business Assets Pledged as Collateral [Member] Business Assets Pledged as Collateral [Member] Payments for (Proceeds from) Productive Assets Payments for (Proceeds from) Productive Assets Loans and Leases Receivable Recorded Investment, Impaired [Line Items] Financing Receivable, Impaired [Line Items] US Treasury Notes Securities US Treasury Notes Securities [Member] Held-to-Maturity Amortized Cost, After Ten Years Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 10 Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract]