Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 79,512	$ (24,623)
Other comprehensive income (loss) before reclassifications	(143,403)	17,891
Reclassification of amounts included in net income	1,002	882
Other comprehensive income (loss), net of tax	(142,401)	18,773
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	(53,354)	(5,850)
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	117,230	16,153
Other comprehensive income (loss) before reclassifications	(143,403)	17,891
Reclassification of amounts included in net income	15	0
Other comprehensive income (loss), net of tax	(143,388)	17,891
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	(8,601)	34,044
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	987	882
Other comprehensive income (loss), net of tax	987	882
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	$ (44,753)	$ (39,894)