Stockholders' Equity (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				$ (120,532)	$ (80,595)
Net change in unrealized gain (loss)		$ 5,949	$ (5,604)	19,794	39,997
Reclassification of net (gain) loss realized and included in earnings		1,374	1,158	4,479	2,960
Valuation adjustment for pension plan amendment	[1]			17,315
Valuation adjustment for employee benefit plans		1,597	(6,347)	(9,185)	(6,347)
Amortization of unrealized net loss on securities transferred to HTM		977	1,108	2,726	2,736
Income tax expense (benefit)		3,609	(3,537)	12,748	14,440
Ending Balance		(98,151)	(55,689)	(98,151)	(55,689)
Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(28,679)	4,268
Net change in unrealized gain (loss)				21,026	38,835
Reclassification of net (gain) loss realized and included in earnings					(1,435)
Income tax expense (benefit)				7,649	13,701
Ending Balance		(15,302)	27,967	(15,302)	27,967
HTM Securities Transferred from AFS [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(14,392)	(16,795)
Amortization of unrealized net loss on securities transferred to HTM				2,726	2,736
Income tax expense (benefit)				1,012	1,029
Ending Balance		(12,678)	(15,088)	(12,678)	(15,088)
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(72,501)	(67,890)
Reclassification of net (gain) loss realized and included in earnings				4,144	4,395
Valuation adjustment for pension plan amendment	[1]			17,315
Valuation adjustment for employee benefit plans				(9,185)	(6,347)
Income tax expense (benefit)				4,416	(714)
Ending Balance		(64,643)	(69,128)	(64,643)	(69,128)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(4,960)	(178)
Net change in unrealized gain (loss)				(1,232)	1,162
Reclassification of net (gain) loss realized and included in earnings				335
Income tax expense (benefit)				(329)	424
Ending Balance		$ (5,528)	$ 560	(5,528)	560
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	[2],[3]			$ (4,144)	$ (4,395)

[1] For further discussion of the pension plan amendment, see Note 11 - Retirement Plans.

[2] Amounts in parenthesis indicate reduction in net income.

[3] 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 11 - Retirement Plans for additional details).