Accumulated Other Comprehensive Income, Net of Tax (Schedule of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 3,174,779	$ 3,065,217	$ 3,207,770	$ 2,886,515
Net current-period OCI	10,592	28,956	88,109	93,418
Ending Balance	3,219,690	3,152,394	3,219,690	3,152,394
Available For Sale and Transferred Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	63,476	(9,406)	17,251	(71,374)
OCI before reclassifications	11,766	24,304	57,997	86,272
Amounts reclassified from AOCI	0	0	(6)	0
Net current-period OCI	11,766	24,304	57,991	86,272
Ending Balance	75,242	14,898	75,242	14,898
Derivative Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	20,649	(8,931)	(9,184)	(9,313)
OCI before reclassifications	(3,310)	2,216	23,655	507
Amounts reclassified from AOCI	1,398	1,387	4,266	3,478
Net current-period OCI	(1,912)	3,603	27,921	3,985
Ending Balance	18,737	(5,328)	18,737	(5,328)
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(42,680)	(47,853)	(44,139)	(49,965)
OCI before reclassifications	0	0	0	0
Amounts reclassified from AOCI	738	1,049	2,197	3,161
Net current-period OCI	738	1,049	2,197	3,161
Ending Balance	(41,942)	(46,804)	(41,942)	(46,804)
Accumulated Other Comprehensive Income, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	41,445	(66,190)	(36,072)	(130,652)
OCI before reclassifications	8,456	26,520	81,652	86,779
Amounts reclassified from AOCI	2,136	2,436	6,457	6,639
Net current-period OCI	10,592	28,956	88,109	93,418
Ending Balance	$ 52,037	$ (37,234)	$ 52,037	$ (37,234)