Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ (842,951)
Other comprehensive income (loss), net of tax	$ 71,851	$ (11,261)	(8,709)	$ 111,532
Ending Balance	(851,660)		(851,660)
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			(9,039)	111,032
Reclassification of amounts included in net income			0	11
Other comprehensive income (loss), net of tax			(9,039)	111,043
Ending Balance	(833,108)	(1,119,785)	(833,108)	(1,119,785)
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			330	489
Other comprehensive income (loss), net of tax			330	489
Ending Balance	(18,552)	(20,687)	(18,552)	(20,687)
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			(9,039)	111,032
Reclassification of amounts included in net income			330	500
Other comprehensive income (loss), net of tax			(8,709)	111,532
Ending Balance	$ (851,660)	$ (1,140,472)	$ (851,660)	$ (1,140,472)