Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income, Net of Tax) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 79,512
Net other comprehensive income (loss) during period	(152,647)	$ 104,135	$ (138,486)
Ending Balance	(63,600)	79,512
Securities Available For Sale [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	117,230	16,153	160,611
Other comprehensive income (loss) before reclassification	(151,013)	97,865	(134,725)
Reclassification of amounts included in net income	123	3,212	(9,733)
Net other comprehensive income (loss) during period	(150,890)	101,077	(144,458)
Reclassification of certain income tax effects related to U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act	17,557
Ending Balance	(16,103)	117,230	16,153
Defined Benefit Plans [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(37,718)	(40,776)	(46,748)
Other comprehensive income (loss) before reclassification	(5,708)	(471)	1,244
Reclassification of amounts included in net income	3,951	3,529	4,728
Net other comprehensive income (loss) during period	(1,757)	3,058	5,972
Reclassification of certain income tax effects related to U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act	(8,022)
Ending Balance	(47,497)	(37,718)	(40,776)
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	79,512	(24,623)	113,863
Other comprehensive income (loss) before reclassification	(156,721)	97,394	(133,481)
Reclassification of amounts included in net income	4,074	6,741	(5,005)
Net other comprehensive income (loss) during period	(152,647)	104,135	(138,486)
Reclassification of certain income tax effects related to U.S. statutory federal income tax rate under the Tax Cuts and Jobs Act	9,535
Ending Balance	$ (63,600)	$ 79,512	$ (24,623)