Stockholders' Equity (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				$ (134,402)	$ (120,532)
Net change in unrealized gain (loss)		$ (23,409)	$ 12,661	(85,653)	13,845
Reclassification of net losses realized and included in earnings		2,217	1,718	4,013	3,105
Valuation adjustment for pension plan amendment			17,315		17,315
Other Valuation adjustment for employee benefit plans		(9,039)	(10,782)	(9,039)	(10,782)
Amortization of unrealized net loss on securities transferred to HTM		925	1,099	1,680	1,749
Income tax expense (benefit)		(6,646)	7,938	(20,192)	9,139
Ending Balance		(203,209)	(104,439)	(203,209)	(104,439)
Accumulcated Other Comprehensive Loss Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(29,512)	(28,679)
Net change in unrealized gain (loss)				(71,759)	14,687
Income tax expense (benefit)				(16,284)	5,333
Ending Balance		(84,987)	(19,325)	(84,987)	(19,325)
Held to Maturity Securities Transferred from AFS [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(14,585)	(14,392)
Amortization of unrealized net loss on securities transferred to HTM				1,680	1,749
Income tax expense (benefit)				380	657
Ending Balance		(13,285)	(13,300)	(13,285)	(13,300)
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(79,078)	(72,501)
Reclassification of net losses realized and included in earnings				2,448	3,105
Valuation adjustment for pension plan amendment					17,315
Other Valuation adjustment for employee benefit plans				(9,039)	(10,782)
Income tax expense (benefit)				(1,494)	3,458
Ending Balance		(84,175)	(66,321)	(84,175)	(66,321)
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(11,227)	(4,960)
Net change in unrealized gain (loss)				(13,894)	(842)
Reclassification of net losses realized and included in earnings				1,565
Income tax expense (benefit)				(2,794)	(309)
Ending Balance		$ (20,762)	$ (5,493)	(20,762)	(5,493)
Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation adjustment for pension plan amendment	[1],[2]			$ (2,448)	$ (3,105)

[1] Amounts in parentheses indicate reduction in net income.

[2] These AOCI components are included in the computation of net periodic pension and post-retirement cost that is reported with employee benefits expense (see Note 12 - Retirement Plans for additional details).