v3.25.2
Investment securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2025
Dec. 31, 2024
Debt Securities, Available-for-Sale [Line Items]    
Investment securities available for sale, amortized cost $ 21,458,000 $ 19,054,000
Investment securities available for sale, gross unrealized gains 178,000 34,000
Investment securities available for sale, gross unrealized losses 96,000 239,000
Investment securities available for sale, estimated fair value 21,540,000 18,849,000
Total debt securities amortized cost 34,482,000 33,249,000
Total debt securities gross unrealized gains 191,000 43,000
Total debt securities gross unrealized losses 1,060,000 1,488,000
Total debt securities estimated fair value 33,613,000 31,804,000
Readily marketable securities amortized cost 273,000 235,000
Readily marketable securities gross unrealized gains 3,000 3,000
Readily marketable securities gross unrealized losses 2,000 3,000
Readily marketable securities estimated fair value 274,000 235,000
Other securities, amortized cost 730,000 772,000
Other securities, estimated fair value 730,000 772,000
Equity and other securities, cost 1,003,000 1,007,000
Equity securities, gross unrealized gains 3,000 3,000
Equity securities, gross unrealized losses 2,000 3,000
Equity and other securities, estimated fair value 1,004,000 1,007,000
Accrued interest and other assets 10,384,000 9,510,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]    
Amortized cost for held to maturity 13,024,000 14,195,000
Gross unrealized gains for held to maturity 13,000 9,000
Gross unrealized losses for held to maturity 964,000 1,249,000
Investment securities, held to maturity, fair value 12,073,000 12,955,000
U.S. Treasury    
Debt Securities, Available-for-Sale [Line Items]    
Investment securities available for sale, amortized cost 7,804,000 7,945,000
Investment securities available for sale, gross unrealized gains 39,000 13,000
Investment securities available for sale, gross unrealized losses 6,000 27,000
Investment securities available for sale, estimated fair value 7,837,000 7,931,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]    
Amortized cost for held to maturity 443,000 1,015,000
Gross unrealized gains for held to maturity 0 0
Gross unrealized losses for held to maturity 8,000 14,000
Investment securities, held to maturity, fair value 435,000 1,001,000
Commercial    
Debt Securities, Available-for-Sale [Line Items]    
Investment securities available for sale, amortized cost 4,708,000 3,739,000
Investment securities available for sale, gross unrealized gains 63,000 8,000
Investment securities available for sale, gross unrealized losses 4,000 45,000
Investment securities available for sale, estimated fair value 4,767,000 3,702,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]    
Amortized cost for held to maturity 2,019,000 2,034,000
Gross unrealized gains for held to maturity 0 0
Gross unrealized losses for held to maturity 94,000 157,000
Investment securities, held to maturity, fair value 1,925,000 1,877,000
Residential    
Debt Securities, Available-for-Sale [Line Items]    
Investment securities available for sale, amortized cost 8,943,000 7,368,000
Investment securities available for sale, gross unrealized gains 76,000 13,000
Investment securities available for sale, gross unrealized losses 86,000 167,000
Investment securities available for sale, estimated fair value 8,933,000 7,214,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]    
Amortized cost for held to maturity 8,301,000 8,773,000
Gross unrealized gains for held to maturity 2,000 0
Gross unrealized losses for held to maturity 760,000 961,000
Investment securities, held to maturity, fair value 7,543,000 7,812,000
Privately issued    
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]    
Amortized cost for held to maturity 34,000 37,000
Gross unrealized gains for held to maturity 11,000 9,000
Gross unrealized losses for held to maturity 0 0
Investment securities, held to maturity, fair value 45,000 46,000
State and political subdivisions    
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]    
Amortized cost for held to maturity 2,226,000 2,335,000
Gross unrealized gains for held to maturity 0 0
Gross unrealized losses for held to maturity 102,000 117,000
Investment securities, held to maturity, fair value 2,124,000 2,218,000
Other    
Debt Securities, Available-for-Sale [Line Items]    
Investment securities available for sale, amortized cost 3,000 2,000
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 3,000 2,000
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 $ 190,000 $ 176,000