Accumulated Other Comprehensive Income, Net of Tax (Schedule of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 3,090,242	$ 2,966,255	$ 3,207,770	$ 2,886,515
Net current-period OCI	66,245	35,651	77,517	64,462
Ending Balance	3,174,779	3,065,217	3,174,779	3,065,217
Available For Sale and Transferred Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	1,562	(43,815)	17,251	(71,374)
OCI before reclassifications	61,914	34,409	46,231	61,968
Amounts reclassified from AOCI	0	0	(6)	0
Net current-period OCI	61,914	34,409	46,225	61,968
Ending Balance	63,476	(9,406)	63,476	(9,406)
Derivative Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	17,048	(9,117)	(9,184)	(9,313)
OCI before reclassifications	2,186	(870)	26,965	(1,709)
Amounts reclassified from AOCI	1,415	1,056	2,868	2,091
Net current-period OCI	3,601	186	29,833	382
Ending Balance	20,649	(8,931)	20,649	(8,931)
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(43,410)	(48,909)	(44,139)	(49,965)
OCI before reclassifications	0	0	0	0
Amounts reclassified from AOCI	730	1,056	1,459	2,112
Net current-period OCI	730	1,056	1,459	2,112
Ending Balance	(42,680)	(47,853)	(42,680)	(47,853)
Accumulated Other Comprehensive Income, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(24,800)	(101,841)	(36,072)	(130,652)
OCI before reclassifications	64,100	33,539	73,196	60,259
Amounts reclassified from AOCI	2,145	2,112	4,321	4,203
Net current-period OCI	66,245	35,651	77,517	64,462
Ending Balance	$ 41,445	$ (66,190)	$ 41,445	$ (66,190)