Investment securities - Investment Securities in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	$ 8,730,000	$ 662,000
AFS, less than 12 months, gross unrealized losses	86,000	3,000
AFS, 12 months or longer, fair value	1,165,000	1,554,000
AFS, 12 months or longer, gross unrealized losses	60,000	59,000
Fair Value	9,895,000	2,216,000
Unrealized Losses	146,000	62,000
Total investment securities, less than 12 months, fair value	10,570,000	844,000
Total investment securities, less than 12 months, gross unrealized losses	113,000	4,000
Total investment securities, 12 months or longer, fair value	9,828,000	11,938,000
Total investment securities, 12 months or longer, gross unrealized losses	812,000	795,000
Fair Value	20,398,000	12,782,000
Unrealized Losses	925,000	799,000
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	1,840,000	182,000
HTM, Less than 12 months, gross unrealized losses	27,000	1,000
HTM, 12 months or longer, fair value	8,663,000	10,384,000
HTM, 12 months or longer, gross unrealized losses	752,000	736,000
Fair Value	10,503,000	10,566,000
Unrealized Losses	779,000	737,000
U.S. Treasury
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	248,000	0
AFS, less than 12 months, gross unrealized losses	0	0
AFS, 12 months or longer, fair value	0	185,000
AFS, 12 months or longer, gross unrealized losses	0	2,000
Fair Value	248,000	185,000
Unrealized Losses	0	2,000
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	0	0
HTM, Less than 12 months, gross unrealized losses	0	0
HTM, 12 months or longer, fair value	393,000	391,000
HTM, 12 months or longer, gross unrealized losses	4,000	4,000
Fair Value	393,000	391,000
Unrealized Losses	4,000	4,000
Commercial
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	723,000	174,000
AFS, less than 12 months, gross unrealized losses	3,000	1,000
AFS, 12 months or longer, fair value	1,000	65,000
AFS, 12 months or longer, gross unrealized losses	0	0
Fair Value	724,000	239,000
Unrealized Losses	3,000	1,000
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	274,000	28,000
HTM, Less than 12 months, gross unrealized losses	4,000	0
HTM, 12 months or longer, fair value	1,644,000	1,840,000
HTM, 12 months or longer, gross unrealized losses	74,000	74,000
Fair Value	1,918,000	1,868,000
Unrealized Losses	78,000	74,000
Residential
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	7,759,000	488,000
AFS, less than 12 months, gross unrealized losses	83,000	2,000
AFS, 12 months or longer, fair value	1,163,000	1,303,000
AFS, 12 months or longer, gross unrealized losses	60,000	57,000
Fair Value	8,922,000	1,791,000
Unrealized Losses	143,000	59,000
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	1,010,000	139,000
HTM, Less than 12 months, gross unrealized losses	15,000	1,000
HTM, 12 months or longer, fair value	5,307,000	6,287,000
HTM, 12 months or longer, gross unrealized losses	618,000	618,000
Fair Value	6,317,000	6,426,000
Unrealized Losses	633,000	619,000
Other
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	0	0
AFS, less than 12 months, gross unrealized losses	0	0
AFS, 12 months or longer, fair value	1,000	1,000
AFS, 12 months or longer, gross unrealized losses	0	0
Fair Value	1,000	1,000
Unrealized Losses	0	0
State and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	554,000	13,000
HTM, Less than 12 months, gross unrealized losses	8,000	0
HTM, 12 months or longer, fair value	1,319,000	1,866,000
HTM, 12 months or longer, gross unrealized losses	56,000	40,000
Fair Value	1,873,000	1,879,000
Unrealized Losses	64,000	40,000
Privately issued
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	2,000	2,000
HTM, Less than 12 months, gross unrealized losses	0	0
HTM, 12 months or longer, fair value	0	0
HTM, 12 months or longer, gross unrealized losses	0	0
Fair Value	2,000	2,000
Unrealized Losses	$ 0	$ 0

Investment Securities Continuous Unrealized Loss Position

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Investment Securities Continuous Unrealized Loss Position, Aggregate Losses

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Amount of accumulated unrealized loss on investments in debt and equity securities that have been in a continuous loss position for less than twelve months.

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