Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ (63,600)
Other comprehensive income (loss), net of tax	157,374	$ (142,401)
Ending Balance	93,774
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(16,103)	117,230
Other comprehensive income (loss) before reclassifications	156,263	(143,403)
Reclassification of amounts included in net income	0	15
Other comprehensive income (loss), net of tax	156,263	(143,388)
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	140,160	(8,601)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(47,497)	(37,718)
Other comprehensive income (loss) before reclassifications	0	0
Reclassification of amounts included in net income	1,111	987
Other comprehensive income (loss), net of tax	1,111	987
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	(46,386)	(44,753)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(63,600)	79,512
Other comprehensive income (loss) before reclassifications	156,263	(143,403)
Reclassification of amounts included in net income	1,111	1,002
Other comprehensive income (loss), net of tax	157,374	(142,401)
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	$ 93,774	$ (53,354)