Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 2,966,255	$ 2,716,142	$ 2,886,515	$ 2,701,958
Other comprehensive income (loss), net of tax	35,651	(6,457)	64,462	(30,405)
Ending Balance	3,065,217	2,761,723	3,065,217	2,761,723
Available For Sale and Transferred Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(43,815)	(55,371)	(71,374)	(27,947)
OCI (OCL) before reclassifications	34,409	(9,246)	61,968	(36,670)
Amounts reclassified from AOCL	0	0	0	0
Other comprehensive income (loss), net of tax	34,409	(9,246)	61,968	(36,670)
Ending Balance	(9,406)	(64,617)	(9,406)	(64,617)
Derivative Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(9,117)	(12,494)	(9,313)	(15,016)
OCI (OCL) before reclassifications	(870)	294	(1,709)	1,423
Amounts reclassified from AOCL	1,056	1,386	2,091	2,779
Other comprehensive income (loss), net of tax	186	1,680	382	4,202
Ending Balance	(8,931)	(10,814)	(8,931)	(10,814)
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(48,909)	(47,614)	(49,965)	(48,568)
OCI (OCL) before reclassifications	0	0	0	0
Amounts reclassified from AOCL	1,056	1,109	2,112	2,063
Other comprehensive income (loss), net of tax	1,056	1,109	2,112	2,063
Ending Balance	(47,853)	(46,505)	(47,853)	(46,505)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(101,841)	(115,479)	(130,652)	(91,531)
OCI (OCL) before reclassifications	33,539	(8,952)	60,259	(35,247)
Amounts reclassified from AOCL	2,112	2,495	4,203	4,842
Other comprehensive income (loss), net of tax	35,651	(6,457)	64,462	(30,405)
Ending Balance	$ (66,190)	$ (121,936)	$ (66,190)	$ (121,936)