Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 79,512	$ (24,623)
Other comprehensive income (loss) before reclassifications			(154,403)	74,136
Reclassification of amounts included in net income			2,037	1,797
Other comprehensive income (loss), net of tax	$ (9,965)	$ 57,160	(152,366)	75,933
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings			9,535
Ending Balance	(63,319)	51,310	(63,319)	51,310
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			117,230	16,153
Other comprehensive income (loss) before reclassifications			(154,403)	74,136
Reclassification of amounts included in net income			62	32
Other comprehensive income (loss), net of tax			(154,341)	74,168
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings			17,557
Ending Balance	(19,554)	90,321	(19,554)	90,321
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(37,718)	(40,776)
Other comprehensive income (loss) before reclassifications			0	0
Reclassification of amounts included in net income			1,975	1,765
Other comprehensive income (loss), net of tax			1,975	1,765
Reclassification of certain income tax effects related to the change in the U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act to retained earnings			(8,022)
Ending Balance	$ (43,765)	$ (39,011)	$ (43,765)	$ (39,011)