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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 2,527,012	$ 2,413,960
Other comprehensive income (loss), net of tax	$ 2,984	$ 4,358	7,895	24,953
Ending Balance	2,638,787	2,511,629	2,638,787	2,511,629
Available For Sale and Transferred Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(14,501)	12,363	(15,476)	(6,407)
OCI/OCL before reclassifications	872	1,218	1,847	20,156
Amounts reclassified from AOCL	0	0	0	(168)
Other comprehensive income (loss), net of tax	872	1,218	1,847	19,988
Ending Balance	(13,629)	13,581	(13,629)	13,581
Derivative Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(15,258)	(23,406)	(17,068)	(22,980)
OCI/OCL before reclassifications	(34)	794	(445)	(2,416)
Amounts reclassified from AOCL	1,145	1,221	3,366	4,005
Other comprehensive income (loss), net of tax	1,111	2,015	2,921	1,589
Ending Balance	(14,147)	(21,391)	(14,147)	(21,391)
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(42,323)	(46,468)	(44,449)	(48,719)
OCI/OCL before reclassifications	0	0	0	0
Amounts reclassified from AOCL	1,001	1,125	3,127	3,376
Other comprehensive income (loss), net of tax	1,001	1,125	3,127	3,376
Ending Balance	(41,322)	(45,343)	(41,322)	(45,343)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(72,082)	(57,511)	(76,993)	(78,106)
OCI/OCL before reclassifications	838	2,012	1,402	17,740
Amounts reclassified from AOCL	2,146	2,346	6,493	7,213
Other comprehensive income (loss), net of tax	2,984	4,358	7,895	24,953
Ending Balance	$ (69,098)	$ (53,153)	$ (69,098)	$ (53,153)