Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 79,512	$ (24,623)
Other comprehensive income (loss) before reclassifications			(233,334)	76,455
Reclassification of amounts included in net income			3,052	5,843
Other comprehensive income (loss), net of tax	$ (77,916)	$ 6,365	(230,282)	82,298
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	(141,235)	57,675	(141,235)	57,675
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			117,230	16,153
Other comprehensive income (loss) before reclassifications			(233,334)	76,455
Reclassification of amounts included in net income			89	3,196
Other comprehensive income (loss), net of tax			(233,245)	79,651
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	(98,458)	95,804	(98,458)	95,804
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			2,963	2,647
Other comprehensive income (loss), net of tax			2,963	2,647
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	$ (42,777)	$ (38,129)	$ (42,777)	$ (38,129)