Investment securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	$ 23,000,000	$ 19,054,000
Investment securities available for sale, gross unrealized gains	236,000	34,000
Investment securities available for sale, gross unrealized losses	73,000	239,000
Investment securities available for sale, estimated fair value	23,163,000	18,849,000
Total debt securities amortized cost	35,711,000	33,249,000
Total debt securities gross unrealized gains	255,000	43,000
Total debt securities gross unrealized losses	879,000	1,488,000
Total debt securities estimated fair value	35,087,000	31,804,000
Readily marketable securities amortized cost	257,000	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	258,000	235,000
Other securities, amortized cost	732,000	772,000
Other securities, estimated fair value	732,000	772,000
Equity and other securities, cost	989,000	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	990,000	1,007,000
Accrued interest and other assets	10,644,000	9,510,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	12,711,000	14,195,000
Gross unrealized gains for held to maturity	19,000	9,000
Gross unrealized losses for held to maturity	806,000	1,249,000
Investment securities, held to maturity, fair value	11,924,000	12,955,000
U.S. Treasury
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	6,581,000	7,945,000
Investment securities available for sale, gross unrealized gains	42,000	13,000
Investment securities available for sale, gross unrealized losses	3,000	27,000
Investment securities available for sale, estimated fair value	6,620,000	7,931,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	444,000	1,015,000
Gross unrealized gains for held to maturity	0	0
Gross unrealized losses for held to maturity	6,000	14,000
Investment securities, held to maturity, fair value	438,000	1,001,000
Commercial
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	4,789,000	3,739,000
Investment securities available for sale, gross unrealized gains	74,000	8,000
Investment securities available for sale, gross unrealized losses	2,000	45,000
Investment securities available for sale, estimated fair value	4,861,000	3,702,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	2,020,000	2,034,000
Gross unrealized gains for held to maturity	0	0
Gross unrealized losses for held to maturity	77,000	157,000
Investment securities, held to maturity, fair value	1,943,000	1,877,000
Residential
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	11,628,000	7,368,000
Investment securities available for sale, gross unrealized gains	120,000	13,000
Investment securities available for sale, gross unrealized losses	68,000	167,000
Investment securities available for sale, estimated fair value	11,680,000	7,214,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	8,048,000	8,773,000
Gross unrealized gains for held to maturity	7,000	0
Gross unrealized losses for held to maturity	662,000	961,000
Investment securities, held to maturity, fair value	7,393,000	7,812,000
Privately issued
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	33,000	37,000
Gross unrealized gains for held to maturity	12,000	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	2,165,000	2,335,000
Gross unrealized gains for held to maturity	0	0
Gross unrealized losses for held to maturity	61,000	117,000
Investment securities, held to maturity, fair value	2,104,000	2,218,000
Other
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	2,000	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	2,000	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		$ 176,000