Other Comprehensive Income (Loss) (Changes in Accumulated Other Comprehensive Income (Loss) by Component (Net of Income Taxes)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Changes in Accumulated Other Comprehensive Income (Loss) by Component [Roll Forward]
Balance	$ 2,961,566	$ 2,927,924
Other comprehensive income (loss), Net of Tax Amount	(45,552)	908
Balance	2,956,495	2,962,127
Total
Changes in Accumulated Other Comprehensive Income (Loss) by Component [Roll Forward]
Balance	(54,754)	(55,659)
Other comprehensive income before reclassifications	(45,531)	5,596
Amounts reclassified from accumulated other comprehensive income (loss)	(21)	(4,688)
Reclassification from adoption of ASU 2018-02	(7,588)
Cumulative-effect adjustment from adoption of ASU 2016-01	117
Other comprehensive income (loss), Net of Tax Amount	(45,552)	908
Balance	(107,777)	(54,751)
Net unrealized gains (losses) on cash flow hedges
Changes in Accumulated Other Comprehensive Income (Loss) by Component [Roll Forward]
Balance	(12,137)	(12,217)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	40
Reclassification from adoption of ASU 2018-02	0
Cumulative-effect adjustment from adoption of ASU 2016-01	0
Other comprehensive income (loss), Net of Tax Amount	0	40
Balance	(12,137)	(12,177)
Net unrealized gains (losses) on investment securities available for sale
Changes in Accumulated Other Comprehensive Income (Loss) by Component [Roll Forward]
Balance	(43,470)	(44,324)
Other comprehensive income before reclassifications	(45,531)	5,596
Amounts reclassified from accumulated other comprehensive income (loss)	0	(4,716)
Reclassification from adoption of ASU 2018-02	(7,763)
Cumulative-effect adjustment from adoption of ASU 2016-01	117
Other comprehensive income (loss), Net of Tax Amount	(45,531)	880
Balance	(96,647)	(43,444)
Post-retirement unfunded health benefit
Changes in Accumulated Other Comprehensive Income (Loss) by Component [Roll Forward]
Balance	853	882
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(21)	(12)
Reclassification from adoption of ASU 2018-02	(175)
Cumulative-effect adjustment from adoption of ASU 2016-01	0
Other comprehensive income (loss), Net of Tax Amount	(21)	(12)
Balance	$ 1,007	$ 870