Entity tax identification number Entity Tax Identification Number Total investments at fair value EBP, Investment, Fair Value Rollover contributions EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover Maximum Maximum [Member] Legal Entity [Axis] Legal Entity [Axis] Minimum pretax compensation contribution EBP, Participant Contribution, Pretax, Minimum Annual Compensation to Total Compensation, Percentage EBP, Participant Contribution, Pretax, Minimum Annual Compensation to Total Compensation, Percentage Statistical Measurement [Domain] Statistical Measurement [Domain] EBP, Description of Plan [Table] EBP, Description of Plan [Table] Mutual funds Mutual Fund [Member] EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Related Party and Party-in-Interest Transactions [Table] Maximum pretax compensation contribution EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage Total deductions EBP, Change in Net Asset Available for Benefit, Decrease Maximum borrowing, amount EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount Fair Value of Financial Instruments EBP, Investment, Fair Value and NAV [Text Block] Plan's Interest in Master Trust Balances EBP, Nonconsolidated, Plan Interest in Master Trust [Member] Level 1 Fair Value, Inputs, Level 1 [Member] EBP, Investment, Fair Value and NAV [Abstract] Statistical Measurement [Axis] Statistical Measurement [Axis] Additions EBP, Change in Net Asset Available for Benefit, Increase [Abstract] EBP, Statement of Change in Net Asset Available for Benefit [Table] EBP, Statement of Change in Net Asset Available for Benefit [Table] Participant Distributions and Withdrawals EBP, Payment to Participant [Policy Text Block] Transfers to GATX Corporation Salaried Employees Retirement Savings Plan EBP, Change in Net Asset Available for Benefit, Transfer to (from) Plan EBP, Master Trust [Line Items] EBP, Master Trust [Line Items] Amendment Flag Amendment Flag Basis of Presentation EBP, Basis of Accounting [Policy Text Block] EBP, Schedule of Asset Held for Investment [Table] EBP, Schedule of Asset Held for Investment [Table] Plus: EBP, Receivable [Abstract] EBP, Description of Plan and Accounting Change [Abstract] Fair Value of GATX Corporation Master Trust's Assets EBP, Investment, Fair Value and NAV [Table Text Block] Investments [Domain] Investments [Domain] EBP, Master Trust [Abstract] Use of Estimates EBP, Use of Estimate [Policy Text Block] Schedule H, Line 4i – Schedule of Assets (Held At End of Year) EBP, Schedule of Asset Held for Investment [Text Block] Cover [Abstract] Risks and Uncertainties EBP, Risk and Uncertainty [Text Block] Employer contributions EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash Catch-up contributions, percentage EBP, Participant Contribution, Pretax, Annual Compensation to Total Compensation, Catch Up Contributions, Percentage EBP, Participant Contribution, Pretax, Annual Compensation to Total Compensation, Catch Up Contributions, Percentage Minimum Minimum [Member] Investment Type [Axis] Investment Type [Axis] Interest income on notes receivable from participants EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Note Receivable from Participant Total contributions EBP, Change in Net Asset Available for Benefit, Increase from Contribution Participant contributions EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Excluding Rollover Net appreciation in fair value of investments EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Gain (Loss) on Investment Interest and dividend income EBP, Change in Net Asset Available for Benefit, Increase from Interest and Dividend Income on Investment EBP, Description of Plan [Line Items] EBP, Description of Plan [Line Items] Plan number EBP, Plan Number EBP, Investment, Fair Value and NAV [Line Items] EBP, Investment, Fair Value and NAV [Line Items] Description of the Plan EBP, Description of Plan [Text Block] EBP, Statement of Change in Net Asset Available for Benefit [Line Items] EBP, Statement of Change in Net Asset Available for Benefit [Line Items] EBP, Statement of Net Asset Available for Benefit [Table] EBP, Statement of Net Asset Available for Benefit [Table] EBP, Schedule of Asset Held for Investment [Line Items] EBP, Schedule of Asset Held for Investment [Line Items] Contributions EBP, Contribution [Policy Text Block] EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Domain] EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Domain] EBP, Schedule of Asset Held for Investment [Abstract] EBP, Accounting Policy [Table] EBP, Accounting Policy [Table] Total additions EBP, Change in Net Asset Available for Benefit, Increase Allowance for credit losses EBP, Note Receivable from Participant, Allowance for Credit Loss Administrative fees EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense Document Information [Line Items] Document Information [Line Items] EBP, Risk and Uncertainty [Line Items] EBP, Risk and Uncertainty [Line Items] EBP, Risk and Uncertainty [Table] EBP, Risk and Uncertainty [Table] GATX Corporation Master Trust EBP, Master Trust [Text Block] Percentage of eligible compensation matched EBP, Employer Contribution, Participant Compensation Matched, Percentage EBP, Statement of Change in Net Asset Available for Benefit [Abstract] Level 2 Fair Value, Inputs, Level 2 [Member] Involuntary cash-out limit EBP, Involuntary Cash-out Limit EBP, Involuntary Cash-out Limit EBP, Tax Status [Table] EBP, Tax Status [Table] Net increase EBP, Change in Net Asset Available for Benefit, Increase (Decrease) Related Party and Party-in-Interest Transactions EBP, Related Party and Party-in-Interest Transactions [Text Block] EBP, Related Party and Party-in-Interest Transactions [Abstract] EBP, Accounting Policy, Accounting Standard Update, and Change in Accounting Principle [Abstract] Participant distributions and withdrawals EBP, Change in Net Asset Available for Benefit, Decrease from Payment to Participant Plan interest in the GATX Corporation Master Trust EBP, Investment, Plan Interest in Master Trust, Fair Value Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] EBP, Statement of Net Asset Available for Benefit [Abstract] EBP, Accounting Policy [Line Items] EBP, Accounting Policy [Line Items] EBP, Tax Status [Abstract] GATX common stock fund EBP, Employer, Common Stock Fund [Member] Net investment income from plan interest in the GATX Corporation Master Trust EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Change in Plan Interest in Master Trust Participant loans (3.50% to 8.25% interest rates, various maturities) EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution Investment Valuation and Income Recognition EBP, Investment [Policy Text Block] EBP, Master Trust [Table] EBP, Master Trust [Table] Document Type Document Type Minimum service period (in hours) EBP, Requisite Service Period, Number Of Hours EBP, Requisite Service Period, Number Of Hours Entity Central Index Key Entity Central Index Key EBP 004 EBP 004 [Member] EBP 004 Interest rate EBP, Asset Held for Investment, Participant Loan, Interest Rate Collective trusts Common Collective Trust [Member] EBP, Tax Status [Line Items] EBP, Tax Status [Line Items] EBP, Risk and Uncertainty [Abstract] Deductions EBP, Change in Net Asset Available for Benefit, Decrease [Abstract] Notes receivable from participants EBP, Note Receivable from Participant Master Trust Balances EBP, Nonconsolidated, Master Trust [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Significant Accounting Policies EBP, Summary of Accounting Policy [Text Block] Entity Registrant Name Entity Registrant Name Schedule of Master Trust Net Assets EBP, Master Trust [Table Text Block] Income Tax Status EBP, Tax Status [Text Block] Maximum borrowing, to total account, percentage EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage Document Information [Table] Document Information [Table] Notes Receivable from Participants EBP, Note Receivable from Participant [Policy Text Block] Total investment income EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment Vesting percentage EBP, Contribution, Vesting Percentage EBP, Contribution, Vesting Percentage EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Axis] EBP, Nonconsolidated, Master Trust and Plan Interest in Master Trust [Axis] EBP, Related Party and Party-in-Interest Transactions [Line Items] EBP, Related Party and Party-in-Interest Transactions [Line Items] Level 3 Fair Value, Inputs, Level 3 [Member] Other receivables EBP, Other Receivable Net assets available for benefits Net assets available for benefits at beginning of year Net assets available for benefits at end of year EBP, Net Asset Available for Benefit Entity [Domain] Entity [Domain] Contributions EBP, Change in Net Asset Available for Benefit, Increase from Contribution [Abstract] Participant compensation matched, percentage EBP, Employer Contribution, Matching Percentage EBP, Investment, Fair Value and NAV [Table] EBP, Investment, Fair Value and NAV [Table] EBP, Statement of Net Asset Available for Benefit [Line Items] EBP, Statement of Net Asset Available for Benefit [Line Items]