Segment information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Net Interest Income	$ 1,761,000	$ 1,726,000	$ 5,169,000	$ 5,124,000
Noninterest income	752,000	606,000	2,046,000	1,770,000
Provision for credit losses	125,000	120,000	380,000	470,000
Provision for credit losses	110,000	120,000	345,000	470,000
Salaries and employee benefits	833,000	775,000	2,533,000	2,372,000
Depreciation and amortization			243,000	235,000
Income before taxes	1,025,000	909,000	2,721,000	2,428,000
Income tax expense (benefit)	233,000	188,000	629,000	521,000
Net income (loss)	792,000	721,000	2,092,000	1,907,000
Operating Segments
Segment Reporting Information [Line Items]
Net Interest Income	1,761,000	1,726,000	5,169,000	5,124,000
Noninterest income	752,000	606,000	2,046,000	1,770,000
Total revenue	2,513,000	2,332,000	7,215,000	6,894,000
Salaries and employee benefits	833,000	775,000	2,533,000	2,372,000
Depreciation and amortization	122,000	122,000	376,000	379,000
Other direct expenses	408,000	406,000	1,205,000	1,245,000
Indirect expense	0	0	0	0
Income before taxes	1,025,000	909,000	2,721,000	2,428,000
Income tax expense (benefit)	233,000	188,000	629,000	521,000
Average total assets	211,053,000	209,581,000	209,889,000	211,008,000
Commercial Bank | Operating Segments
Segment Reporting Information [Line Items]
Net Interest Income	539,000	551,000	1,599,000	1,652,000
Noninterest income	209,000	172,000	587,000	487,000
Total revenue	748,000	723,000	2,186,000	2,139,000
Provision for credit losses			168,000	193,000
Provision for credit losses	72,000	39,000
Salaries and employee benefits	160,000	152,000	461,000	452,000
Depreciation and amortization	11,000	10,000	32,000	29,000
Other direct expenses	68,000	69,000	208,000	203,000
Indirect expense	127,000	127,000	379,000	378,000
Income before taxes	310,000	326,000	938,000	884,000
Income tax expense (benefit)	82,000	87,000	248,000	239,000
Net income (loss)	228,000	239,000	690,000	645,000
Average total assets	78,227,000	80,434,000	78,691,000	80,903,000
Retail Bank | Operating Segments
Segment Reporting Information [Line Items]
Net Interest Income	999,000	1,087,000	2,959,000	3,248,000
Noninterest income	249,000	205,000	691,000	606,000
Total revenue	1,248,000	1,292,000	3,650,000	3,854,000
Provision for credit losses			220,000	202,000
Provision for credit losses	70,000	74,000
Salaries and employee benefits	204,000	196,000	601,000	577,000
Depreciation and amortization	59,000	61,000	181,000	189,000
Other direct expenses	116,000	105,000	310,000	298,000
Indirect expense	297,000	255,000	868,000	751,000
Income before taxes	502,000	601,000	1,470,000	1,837,000
Income tax expense (benefit)	126,000	155,000	372,000	473,000
Net income (loss)	376,000	446,000	1,098,000	1,364,000
Average total assets	57,653,000	53,128,000	56,022,000	52,771,000
Institutional Services and Wealth Management | Operating Segments
Segment Reporting Information [Line Items]
Net Interest Income	163,000	185,000	500,000	565,000
Noninterest income	244,000	204,000	678,000	602,000
Total revenue	407,000	389,000	1,178,000	1,167,000
Provision for credit losses			5,000	6,000
Provision for credit losses	0	3,000
Salaries and employee benefits	112,000	103,000	325,000	306,000
Depreciation and amortization	2,000	1,000	6,000	6,000
Other direct expenses	25,000	24,000	77,000	75,000
Indirect expense	78,000	74,000	241,000	223,000
Income before taxes	190,000	184,000	524,000	551,000
Income tax expense (benefit)	48,000	47,000	133,000	142,000
Net income (loss)	142,000	137,000	391,000	409,000
Average total assets	4,355,000	3,898,000	4,244,000	3,735,000
All Other | Operating Segments
Segment Reporting Information [Line Items]
Net Interest Income	60,000	(97,000)	111,000	(341,000)
Noninterest income	50,000	25,000	90,000	75,000
Total revenue	110,000	(72,000)	201,000	(266,000)
Provision for credit losses			(13,000)	69,000
Provision for credit losses	(17,000)	4,000
Salaries and employee benefits	357,000	324,000	1,146,000	1,037,000
Depreciation and amortization	50,000	50,000	157,000	155,000
Other direct expenses	199,000	208,000	610,000	669,000
Indirect expense	(502,000)	(456,000)	(1,488,000)	(1,352,000)
Income before taxes	23,000	(202,000)	(211,000)	(844,000)
Income tax expense (benefit)	(23,000)	(101,000)	(124,000)	(333,000)
Net income (loss)	46,000	(101,000)	(87,000)	(511,000)
Average total assets	$ 70,818,000	$ 72,121,000	$ 70,932,000	$ 73,599,000