Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 267,370
Other comprehensive income (loss), net of tax	$ 16,881	$ 85,945	237,775	$ 368,983
Ending Balance	505,145		505,145
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			313,304	(16,103)
Other comprehensive income (loss) before reclassifications			320,724	365,861
Reclassification of amounts included in net income			(86,101)	(210)
Other comprehensive income (loss), net of tax			234,623	365,651
Ending Balance	547,927	349,548	547,927	349,548
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			3,152	3,332
Other comprehensive income (loss), net of tax			3,152	3,332
Ending Balance	(42,782)	(44,165)	(42,782)	(44,165)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			267,370	(63,600)
Other comprehensive income (loss) before reclassifications			320,724	365,861
Reclassification of amounts included in net income			(82,949)	3,122
Other comprehensive income (loss), net of tax			237,775	368,983
Ending Balance	$ 505,145	$ 305,383	$ 505,145	$ 305,383