Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 2,527,012	$ 2,413,960	$ 2,322,815
Other comprehensive income (loss), net of tax	1,110	1,113	(21,845)
Ending Balance	2,701,958	2,527,012	2,413,960
Available For Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(4,881)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(7,590)	(8,901)	(22,512)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(15,476)	(6,407)	16,421
Amounts reclassified from accumulated other comprehensive (loss) income	0	(168)	(316)
Other comprehensive income (loss), net of tax	(7,590)	(9,069)	(22,828)
Ending Balance	(27,947)	(15,476)	(6,407)
Derivative Instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(2,513)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	181	825	(3,136)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(17,068)	(22,980)	(25,530)
Amounts reclassified from accumulated other comprehensive (loss) income	4,384	5,087	5,686
Other comprehensive income (loss), net of tax	4,565	5,912	2,550
Ending Balance	(15,016)	(17,068)	(22,980)
Defined Benefit Pension and Postretirement Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(8,254)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	98	(232)	(5,500)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(44,449)	(48,719)	(47,152)
Amounts reclassified from accumulated other comprehensive (loss) income	4,037	4,502	3,933
Other comprehensive income (loss), net of tax	4,135	4,270	(1,567)
Ending Balance	(48,568)	(44,449)	(48,719)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(15,648)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(7,311)	(8,308)	(31,148)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(76,993)	(78,106)	(56,261)
Amounts reclassified from accumulated other comprehensive (loss) income	8,421	9,421	9,303
Other comprehensive income (loss), net of tax	1,110	1,113	(21,845)
Ending Balance	$ (91,531)	$ (76,993)	$ (78,106)