v3.26.1
Investment securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
 1 Months Ended
Jul. 31, 2026
Jun. 30, 2026
Dec. 31, 2025
Debt Securities, Available-for-Sale [Line Items]      
Investment securities available for sale, amortized cost   $ 25,495,000 $ 22,994,000
Investment securities available for sale, gross unrealized gains   87,000 270,000
Investment securities available for sale, gross unrealized losses   212,000 62,000
Investment securities available for sale, estimated fair value   25,370,000 23,202,000
Total debt securities amortized cost   37,403,000 35,424,000
Total debt securities gross unrealized gains   99,000 292,000
Total debt securities gross unrealized losses   1,013,000 799,000
Total debt securities estimated fair value   36,489,000 34,917,000
Readily marketable securities amortized cost   242,000 280,000
Readily marketable securities gross unrealized gains   5,000 3,000
Readily marketable securities gross unrealized losses   2,000 2,000
Readily marketable securities estimated fair value   245,000 281,000
Other securities, amortized cost   851,000 736,000
Other securities, estimated fair value   851,000 736,000
Equity and other securities, cost   1,093,000 1,016,000
Equity securities, gross unrealized gains   5,000 3,000
Equity securities, gross unrealized losses   2,000 2,000
Equity and other securities, estimated fair value   1,096,000 1,017,000
Accrued interest and other assets   12,193,000 11,348,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]      
Amortized cost for held to maturity   11,908,000 12,430,000
Gross unrealized gains for held to maturity   12,000 22,000
Gross unrealized losses for held to maturity   801,000 737,000
Investment securities, held to maturity, fair value   11,119,000 11,715,000
U.S. Treasury      
Debt Securities, Available-for-Sale [Line Items]      
Investment securities available for sale, amortized cost   3,227,000 6,302,000
Investment securities available for sale, gross unrealized gains   4,000 43,000
Investment securities available for sale, gross unrealized losses   4,000 2,000
Investment securities available for sale, estimated fair value   3,227,000 6,343,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]      
Amortized cost for held to maturity   398,000 445,000
Gross unrealized gains for held to maturity   0 0
Gross unrealized losses for held to maturity   3,000 4,000
Investment securities, held to maturity, fair value   395,000 441,000
Commercial      
Debt Securities, Available-for-Sale [Line Items]      
Investment securities available for sale, amortized cost   4,744,000 4,738,000
Investment securities available for sale, gross unrealized gains   22,000 79,000
Investment securities available for sale, gross unrealized losses   11,000 1,000
Investment securities available for sale, estimated fair value   4,755,000 4,816,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]      
Amortized cost for held to maturity   1,996,000 2,007,000
Gross unrealized gains for held to maturity   0 0
Gross unrealized losses for held to maturity   92,000 74,000
Investment securities, held to maturity, fair value   1,904,000 1,933,000
Residential (b)      
Debt Securities, Available-for-Sale [Line Items]      
Investment securities available for sale, amortized cost   17,523,000 11,953,000
Investment securities available for sale, gross unrealized gains   61,000 148,000
Investment securities available for sale, gross unrealized losses   197,000 59,000
Investment securities available for sale, estimated fair value   17,387,000 12,042,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]      
Amortized cost for held to maturity   7,463,000 7,816,000
Gross unrealized gains for held to maturity   1,000 10,000
Gross unrealized losses for held to maturity   663,000 619,000
Investment securities, held to maturity, fair value   6,801,000 7,207,000
Residential (b) | Subsequent Event      
Debt Securities, Available-for-Sale [Line Items]      
Available-for-sale securities transferred to held-to-maturity $ 8,300,000    
Available-for-sale, transfer to held-to-maturity, gain 32,000    
Available-for-sale, transfer to held-to-maturity, loss $ 24,000    
Privately issued      
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]      
Amortized cost for held to maturity   28,000 32,000
Gross unrealized gains for held to maturity   11,000 12,000
Gross unrealized losses for held to maturity   0 0
Investment securities, held to maturity, fair value   39,000 44,000
State and political subdivisions      
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]      
Amortized cost for held to maturity   2,022,000 2,129,000
Gross unrealized gains for held to maturity   0 0
Gross unrealized losses for held to maturity   43,000 40,000
Investment securities, held to maturity, fair value   1,979,000 2,089,000
Other      
Debt Securities, Available-for-Sale [Line Items]      
Investment securities available for sale, amortized cost   1,000 1,000
Investment securities available for sale, gross unrealized gains   0 0
Investment securities available for sale, gross unrealized losses   0 0
Investment securities available for sale, estimated fair value   1,000 1,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]      
Amortized cost for held to maturity   1,000 1,000
Gross unrealized gains for held to maturity   0 0
Gross unrealized losses for held to maturity   0 0
Investment securities, held to maturity, fair value   1,000 1,000
Debt Securities      
Debt Securities, Available-for-Sale [Line Items]      
Accrued interest and other assets   $ 173,000 $ 187,000