Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 512,970
Other comprehensive income (loss), net of tax	$ (62,707)	$ 16,881	(162,687)	$ 237,775
Ending Balance	350,283		350,283
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			563,801	313,304
Other comprehensive income (loss) before reclassifications			(166,311)	320,724
Reclassification of amounts included in net income			0	(86,101)
Other comprehensive income (loss), net of tax			(166,311)	234,623
Ending Balance	397,490	547,927	397,490	547,927
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(50,831)	(45,934)
Other comprehensive income (loss) before reclassifications			0	0
Reclassification of amounts included in net income			3,624	3,152
Other comprehensive income (loss), net of tax			3,624	3,152
Ending Balance	(47,207)	(42,782)	(47,207)	(42,782)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			512,970	267,370
Other comprehensive income (loss) before reclassifications			(166,311)	320,724
Reclassification of amounts included in net income			3,624	(82,949)
Other comprehensive income (loss), net of tax			(162,687)	237,775
Ending Balance	$ 350,283	$ 505,145	$ 350,283	$ 505,145