Stockholders' Equity (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				$ (134,402)	$ (120,532)
Net change in unrealized gain (loss)		$ (25,242)	$ 5,949	(110,895)	19,794
Reclassification of net losses realized and included in earnings		2,547	1,374	6,560	4,479
Valuation adjustment for pension plan amendment					17,315
Other valuation adjustments for employee benefit plans			1,597	(9,039)	(9,185)
Amortization of unrealized net loss on securities transferred to HTM		747	977	2,427	2,726
Income tax expense (benefit)		(4,978)	3,609	(25,170)	12,748
Ending Balance		(220,179)	(98,151)	(220,179)	(98,151)
Accumulated 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)				(92,477)	21,026
Income tax expense (benefit)				(20,985)	7,649
Ending Balance		(101,004)	(15,302)	(101,004)	(15,302)
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				2,427	2,726
Income tax expense (benefit)				550	1,012
Ending Balance		(12,708)	(12,678)	(12,708)	(12,678)
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				3,719	4,144
Valuation adjustment for pension plan amendment					17,315
Other valuation adjustments for employee benefit plans				(9,039)	(9,185)
Income tax expense (benefit)				(1,205)	4,416
Ending Balance		(83,193)	(64,643)	(83,193)	(64,643)
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)				(18,418)	(1,232)
Reclassification of net losses realized and included in earnings				2,841	335
Income tax expense (benefit)				(3,530)	(329)
Ending Balance		$ (23,274)	$ (5,528)	(23,274)	(5,528)
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]			$ (3,719)	$ (4,144)

[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).