Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 267,370
Other comprehensive income (loss), net of tax	$ 124,449	$ 125,664	220,894	$ 283,038
Ending Balance	488,264		488,264
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			313,304	(16,103)
Other comprehensive income (loss) before reclassifications			304,893	280,951
Reclassification of amounts included in net income			(86,101)	(134)
Other comprehensive income (loss), net of tax			218,792	280,817
Ending Balance	532,096	264,714	532,096	264,714
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(45,934)	(47,497)
Other comprehensive income (loss) before reclassifications			0	0
Reclassification of amounts included in net income			2,102	2,221
Other comprehensive income (loss), net of tax			2,102	2,221
Ending Balance	(43,832)	(45,276)	(43,832)	(45,276)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			267,370	(63,600)
Other comprehensive income (loss) before reclassifications			304,893	280,951
Reclassification of amounts included in net income			(83,999)	2,087
Other comprehensive income (loss), net of tax			220,894	283,038
Ending Balance	$ 488,264	$ 219,438	$ 488,264	$ 219,438