Stockholders' Equity (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ (120,532)
Net change in unrealized gain (loss)		(425)	$ (57,346)	$ (21,270)
Reclassification of net (gain) loss realized and included in earnings		5,801	4,016	3,010
Valuation adjustment for pension plan amendment		17,315
Other valuation adjustments for employee benefit plans		(10,929)	(12,748)	(33,971)
Amortization of unrealized net loss on securities transferred to held to maturity		3,786	3,830	3,530
Income tax expense (benefit)		(4,088)	22,311	18,180
Reclassification of certain tax effects
Ending Balance		(134,402)	(120,532)
Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(28,679)	4,268	18,001
Net change in unrealized gain (loss)		6,903	(49,839)	(21,581)
Reclassification of net (gain) loss realized and included in earnings			(1,912)	(165)
Valuation adjustment for pension plan amendment
Valuation adjustment for employee benefit plans
Other valuation adjustments for employee benefit plans
Amortization of unrealized net loss on securities transferred to held to maturity
Income tax expense (benefit)		1,067	(18,804)	(8,013)
Reclassification of certain tax effects	[1]	(6,669)
Ending Balance		(29,512)	(28,679)	4,268
HTM Securities Transferred from AFS [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(14,392)	(16,795)	(19,074)
Net change in unrealized gain (loss)
Reclassification of net (gain) loss realized and included in earnings
Valuation adjustment for pension plan amendment
Valuation adjustment for employee benefit plans
Other valuation adjustments for employee benefit plans
Amortization of unrealized net loss on securities transferred to held to maturity		3,786	3,830	3,530
Income tax expense (benefit)		1,393	1,427	1,251
Reclassification of certain tax effects	[1]	(2,586)
Ending Balance		(14,585)	(14,392)	(16,795)
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(72,501)	(67,890)	(48,626)
Net change in unrealized gain (loss)
Reclassification of net (gain) loss realized and included in earnings		5,201	5,928	3,175
Valuation adjustment for pension plan amendment		(17,315)
Valuation adjustment for employee benefit plans			(12,748)	(33,971)
Other valuation adjustments for employee benefit plans		(10,929)
Amortization of unrealized net loss on securities transferred to held to maturity
Income tax expense (benefit)		4,228	(2,209)	(11,532)
Reclassification of certain tax effects	[1]	(13,936)
Ending Balance		(79,078)	(72,501)	(67,890)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(4,960)	(178)	(375)
Net change in unrealized gain (loss)		(7,328)	(7,507)	311
Reclassification of net (gain) loss realized and included in earnings		600
Valuation adjustment for pension plan amendment
Valuation adjustment for employee benefit plans
Other valuation adjustments for employee benefit plans
Amortization of unrealized net loss on securities transferred to held to maturity
Income tax expense (benefit)		(2,600)	(2,725)	114
Reclassification of certain tax effects	[1]	(2,139)
Ending Balance		(11,227)	(4,960)	(178)
Accumulated Other ComprehensiveLoss, Net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(120,532)	(80,595)	(50,074)
Net change in unrealized gain (loss)		(425)	(57,346)	(21,270)
Reclassification of net (gain) loss realized and included in earnings		5,801	4,016	3,010
Valuation adjustment for pension plan amendment		(17,315)
Valuation adjustment for employee benefit plans			(12,748)	(33,971)
Other valuation adjustments for employee benefit plans		(10,929)
Amortization of unrealized net loss on securities transferred to held to maturity		3,786	3,830	3,530
Income tax expense (benefit)		4,088	(22,311)	(18,180)
Reclassification of certain tax effects	[1]	(25,330)
Ending Balance		(134,402)	(120,532)	$ (80,595)
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]	$ (5,201)	$ (5,928)

[1] Represents the reclassification of stranded income tax effects to Retained Earnings upon adoption of ASU 2018-02. The adjustment is discussed in more detail later in this footnote.

[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 footnote 16 for additional information).