Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 512,970
Other comprehensive income (loss), net of tax	$ 25,242	$ 124,449	(99,980)	$ 220,894
Ending Balance	412,990		412,990
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			563,801	313,304
Other comprehensive income (loss) before reclassifications			(102,396)	304,893
Reclassification of amounts included in net income			0	(86,101)
Other comprehensive income (loss), net of tax			(102,396)	218,792
Ending Balance	461,405	532,096	461,405	532,096
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			2,416	2,102
Other comprehensive income (loss), net of tax			2,416	2,102
Ending Balance	(48,415)	(43,832)	(48,415)	(43,832)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			512,970	267,370
Other comprehensive income (loss) before reclassifications			(102,396)	304,893
Reclassification of amounts included in net income			2,416	(83,999)
Other comprehensive income (loss), net of tax			(99,980)	220,894
Ending Balance	$ 412,990	$ 488,264	$ 412,990	$ 488,264