Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income, Net of Tax) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 267,370
Net other comprehensive income (loss) during period	245,600	$ 330,970	$ (152,647)
Ending Balance	512,970	267,370
Securities Available For Sale [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	313,304	(16,103)	117,230
Other comprehensive income (loss) before reclassification	336,598	329,638	(151,013)
Reclassification of amounts included in net income	(86,101)	(231)	123
Net other comprehensive income (loss) during period	250,497	329,407	(150,890)
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			17,557
Ending Balance	563,801	313,304	(16,103)
Defined Benefit Plans [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(45,934)	(47,497)	(37,718)
Other comprehensive income (loss) before reclassification	(9,099)	(2,879)	(5,708)
Reclassification of amounts included in net income	4,202	4,442	3,951
Net other comprehensive income (loss) during period	(4,897)	1,563	(1,757)
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			(8,022)
Ending Balance	(50,831)	(45,934)	(47,497)
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	267,370	(63,600)	79,512
Other comprehensive income (loss) before reclassification	327,499	326,759	(156,721)
Reclassification of amounts included in net income	(81,899)	4,211	4,074
Net other comprehensive income (loss) during period	245,600	330,970	(152,647)
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			9,535
Ending Balance	$ 512,970	$ 267,370	$ (63,600)