Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 3,065,217	$ 2,761,723	$ 2,886,515	$ 2,701,958
Other comprehensive income (loss), net of tax	28,956	(9,628)	93,418	(40,033)
Ending Balance	3,152,394	2,816,198	3,152,394	2,816,198
Available For Sale and Transferred Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(9,406)	(64,617)	(71,374)	(27,947)
OCI before reclassifications	24,304	(13,811)	86,272	(50,481)
Amounts reclassified from AOCL	0	0	0	0
Other comprehensive income (loss), net of tax	24,304	(13,811)	86,272	(50,481)
Ending Balance	14,898	(78,428)	14,898	(78,428)
Derivative Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(8,931)	(10,814)	(9,313)	(15,016)
OCI before reclassifications	2,216	197	507	1,620
Amounts reclassified from AOCL	1,387	1,438	3,478	4,217
Other comprehensive income (loss), net of tax	3,603	1,635	3,985	5,837
Ending Balance	(5,328)	(9,179)	(5,328)	(9,179)
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(47,853)	(46,505)	(49,965)	(48,568)
OCI before reclassifications	0	0	0	0
Amounts reclassified from AOCL	1,049	2,548	3,161	4,611
Other comprehensive income (loss), net of tax	1,049	2,548	3,161	4,611
Ending Balance	(46,804)	(43,957)	(46,804)	(43,957)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(66,190)	(121,936)	(130,652)	(91,531)
OCI before reclassifications	26,520	(13,614)	86,779	(48,861)
Amounts reclassified from AOCL	2,436	3,986	6,639	8,828
Other comprehensive income (loss), net of tax	28,956	(9,628)	93,418	(40,033)
Ending Balance	$ (37,234)	$ (131,564)	$ (37,234)	$ (131,564)