Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 267,370
Other comprehensive income (loss), net of tax	96,445	$ 157,374
Ending Balance	363,815
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	313,304	(16,103)
Other comprehensive income (loss) before reclassifications	181,495	156,263
Reclassification of amounts included in net income	(86,101)	0
Other comprehensive income (loss), net of tax	95,394	156,263
Ending Balance	408,698	140,160
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	1,051	1,111
Other comprehensive income (loss), net of tax	1,051	1,111
Ending Balance	(44,883)	(46,386)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	267,370	(63,600)
Other comprehensive income (loss) before reclassifications	181,495	156,263
Reclassification of amounts included in net income	(85,050)	1,111
Other comprehensive income (loss), net of tax	96,445	157,374
Ending Balance	$ 363,815	$ 93,774