Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	$ 20,807,000	$ 19,054,000
Investment securities available for sale, gross unrealized gains	113,000	34,000
Investment securities available for sale, gross unrealized losses	121,000	239,000
Available for sale (cost: $20,807 at March 31, 2025; $19,054 at December 31, 2024)	20,799,000	18,849,000
Total debt securities Amortized cost	34,159,000	33,249,000
Total debt securities Gross unrealized gains	121,000	43,000
Total debt securities Gross unrealized losses	1,173,000	1,488,000
Total debt securities Estimated fair value	33,107,000	31,804,000
Readily marketable securities Amortized cost	280,000	235,000
Readily marketable securities Gross unrealized gains	2,000	3,000
Readily marketable securities Gross unrealized losses	2,000	3,000
Readily marketable securities Estimated fair value	280,000	235,000
Other securities, Amortized cost	706,000	772,000
Other securities, Estimated fair value	706,000	772,000
Equity and other securities, cost	986,000	1,007,000
Equity securities, Gross Unrealized Gains	2,000	3,000
Equity securities, Gross Unrealized Losses	2,000	3,000
Equity and other securities (cost: $986 at March 31, 2025; $1,007 at December 31, 2024)	986,000	1,007,000
Accrued interest and other assets	9,730,000	9,510,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	13,352,000	14,195,000
Gross unrealized gains for held to maturity	8,000	9,000
Gross unrealized losses for held to maturity	1,052,000	1,249,000
Investment securities, held to maturity, fair value	12,308,000	12,955,000
Accrued interest and other assets	9,730,000	9,510,000
Debt Securities
Debt Securities, Available-for-Sale [Line Items]
Accrued interest and other assets	168,000	176,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Accrued interest and other assets	168,000	176,000
U.S. Treasury
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	8,065,000	7,945,000
Investment securities available for sale, gross unrealized gains	31,000	13,000
Investment securities available for sale, gross unrealized losses	11,000	27,000
Available for sale (cost: $20,807 at March 31, 2025; $19,054 at December 31, 2024)	8,085,000	7,931,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	442,000	1,015,000
Gross unrealized gains for held to maturity	0	0
Gross unrealized losses for held to maturity	10,000	14,000
Investment securities, held to maturity, fair value	432,000	1,001,000
Commercial
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	4,316,000	3,739,000
Investment securities available for sale, gross unrealized gains	35,000	8,000
Investment securities available for sale, gross unrealized losses	11,000	45,000
Available for sale (cost: $20,807 at March 31, 2025; $19,054 at December 31, 2024)	4,340,000	3,702,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	2,035,000	2,034,000
Gross unrealized gains for held to maturity	0	0
Gross unrealized losses for held to maturity	114,000	157,000
Investment securities, held to maturity, fair value	1,921,000	1,877,000
Residential
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	8,422,000	7,368,000
Investment securities available for sale, gross unrealized gains	47,000	13,000
Investment securities available for sale, gross unrealized losses	99,000	167,000
Available for sale (cost: $20,807 at March 31, 2025; $19,054 at December 31, 2024)	8,370,000	7,214,000
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	8,547,000	8,773,000
Gross unrealized gains for held to maturity	0	0
Gross unrealized losses for held to maturity	800,000	961,000
Investment securities, held to maturity, fair value	7,747,000	7,812,000
Other
Debt Securities, Available-for-Sale [Line Items]
Investment securities available for sale, amortized cost	4,000	2,000
Investment securities available for sale, gross unrealized gains	0	0
Investment securities available for sale, gross unrealized losses	0	0
Available for sale (cost: $20,807 at March 31, 2025; $19,054 at December 31, 2024)	4,000	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
State and political subdivisions
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	2,291,000	2,335,000
Gross unrealized gains for held to maturity	0	0
Gross unrealized losses for held to maturity	128,000	117,000
Investment securities, held to maturity, fair value	2,163,000	2,218,000
Privately issued
Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss [Abstract]
Amortized cost for held to maturity	36,000	37,000
Gross unrealized gains for held to maturity	8,000	9,000
Gross unrealized losses for held to maturity	0	0
Investment securities, held to maturity, fair value	$ 44,000	$ 46,000