v3.26.1
Fair Value Accounting - Additional Information (Detail)
 3 Months Ended 12 Months Ended
Mar. 31, 2026
USD ($)
Dec. 31, 2025
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other assets acquired through foreclosure $ 123,000,000 $ 137,000,000
Fair value 18,701,000,000 18,788,000,000
Commitments and contingencies (Note 15)
U.S. Treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 5,610,000,000 5,970,000,000
Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Impaired loans with an allowance recorded $ 10,000,000 $ 15,000,000
Junior Subordinated Debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Instrument, measurement input 0.0571 0.0536
Percentage of LIBOR 2.03% 3.65%
Period of basis point spread 3.68% 1.71%
Junior Subordinated Debt | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Junior subordinated debt $ 70,000,000 $ 71,000,000
Fair Value, Nonrecurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Loans HFI 409,000,000 395,000,000
Other assets acquired through foreclosure 123,000,000 137,000,000
Fair Value, Nonrecurring | Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Junior subordinated debt 1,053,000,000 1,068,000,000
Fair Value, Nonrecurring | Letter of Credit | Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Commitments and contingencies (Note 15) 0 0
Fair Value, Nonrecurring | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Loans HFI 409,000,000 395,000,000
Other assets acquired through foreclosure 123,000,000 137,000,000
Fair Value, Nonrecurring | Significant Unobservable Inputs (Level 3) | Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Junior subordinated debt 85,000,000 87,000,000
Fair Value, Nonrecurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Loans HFI 0 0
Other assets acquired through foreclosure 0 0
Fair Value, Nonrecurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Junior subordinated debt 0 0
Fair Value, Nonrecurring | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Loans HFI 0 0
Other assets acquired through foreclosure 0 0
Fair Value, Nonrecurring | Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Junior subordinated debt 968,000,000 981,000,000
Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 18,701,000,000 18,788,000,000
Junior subordinated debt 70,000,000 71,000,000
Fair Value, Recurring | Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 18,701,000,000 18,788,000,000
Fair Value, Recurring | U.S. Treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 5,610,000,000 5,970,000,000
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0 0
Junior subordinated debt 70,000,000 71,000,000
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 0 0
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 5,638,000,000 5,998,000,000
Junior subordinated debt 0 0
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 5,638,000,000 5,998,000,000
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 5,610,000,000 5,970,000,000
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 13,063,000,000 12,790,000,000
Junior subordinated debt 0 0
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Estimate of Fair Value Measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value 13,063,000,000 12,790,000,000
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value $ 0 $ 0