Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ (842,951)
Other comprehensive income (loss), net of tax	$ (752,533)	$ 216,052	(761,242)	$ 327,584
Ending Balance	(1,604,193)		(1,604,193)
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(824,069)	(1,230,828)
Other comprehensive income (loss) before reclassifications			(767,670)	326,839
Reclassification of amounts included in net income			5,934	11
Other comprehensive income (loss), net of tax			(761,736)	326,850
Ending Balance	(1,585,805)	(903,978)	(1,585,805)	(903,978)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(18,882)	(21,176)
Other comprehensive income (loss) before reclassifications			0	0
Reclassification of amounts included in net income			494	734
Other comprehensive income (loss), net of tax			494	734
Ending Balance	(18,388)	(20,442)	(18,388)	(20,442)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(842,951)	(1,252,004)
Other comprehensive income (loss) before reclassifications			(767,670)	326,839
Reclassification of amounts included in net income			6,428	745
Other comprehensive income (loss), net of tax			(761,242)	327,584
Ending Balance	$ (1,604,193)	$ (924,420)	$ (1,604,193)	$ (924,420)