Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ (1,119,219)
Other comprehensive income (loss), net of tax	$ 393,911	$ (473,800)	203,786	$ (430,533)
Ending Balance	(915,433)		(915,433)
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(1,094,794)	(1,313,791)
Other comprehensive income (loss) before reclassifications			202,808	(432,542)
Reclassification of amounts included in net income			(13)	(52)
Other comprehensive income (loss), net of tax			202,795	(432,594)
Ending Balance	(891,999)	(1,746,385)	(891,999)	(1,746,385)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(24,425)	(34,503)
Other comprehensive income (loss) before reclassifications			0	0
Reclassification of amounts included in net income			991	2,061
Other comprehensive income (loss), net of tax			991	2,061
Ending Balance	(23,434)	(32,442)	(23,434)	(32,442)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(1,119,219)	(1,348,294)
Other comprehensive income (loss) before reclassifications			202,808	(432,542)
Reclassification of amounts included in net income			978	2,009
Other comprehensive income (loss), net of tax			203,786	(430,533)
Ending Balance	$ (915,433)	$ (1,778,827)	$ (915,433)	$ (1,778,827)