Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ (842,951)
Other comprehensive income (loss), net of tax	(80,560)	$ 122,793
Ending Balance	(923,511)
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	(80,725)	122,537
Reclassification of amounts included in net income	0	11
Other comprehensive income (loss), net of tax	(80,725)	122,548
Ending Balance	(904,794)	(1,108,280)
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	165	245
Other comprehensive income (loss), net of tax	165	245
Ending Balance	(18,717)	(20,931)
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	(80,725)	122,537
Reclassification of amounts included in net income	165	256
Other comprehensive income (loss), net of tax	(80,560)	122,793
Ending Balance	$ (923,511)	$ (1,129,211)