Stockholders' Equity (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				$ (120,532)		$ (80,595)
Net change in unrealized gain (loss)		$ 12,661	$ 16,629	13,845		45,601
Reclassification of net (gain) loss realized and included in earnings		1,718	711	3,105		1,802
Valuation adjustment for pension plan amendment		17,315		17,315	[1]
Other valuation adjustments for employee benefit plan		(10,782)		(10,782)
Amortization of unrealized net loss on securities transferred to HTM		1,099	830	1,749		1,628
Income tax expense (benefit)		7,938	6,645	9,139		17,977
Ending Balance		(104,439)	(49,541)	(104,439)		(49,541)
Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(28,679)		4,268
Net change in unrealized gain (loss)				14,687		41,990
Reclassification of net (gain) loss realized and included in earnings						(1,114)
Valuation adjustment for pension plan amendment	[1]
Other valuation adjustments for employee benefit plan
Amortization of unrealized net loss on securities transferred to HTM
Income tax expense (benefit)				5,333		14,970
Ending Balance		(19,325)	30,174	(19,325)		30,174
HTM Securities Transferred from AFS [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(14,392)		(16,795)
Net change in unrealized gain (loss)
Reclassification of net (gain) loss realized and included in earnings
Valuation adjustment for pension plan amendment	[1]
Other valuation adjustments for employee benefit plan
Amortization of unrealized net loss on securities transferred to HTM				1,749		1,628
Income tax expense (benefit)				657		625
Ending Balance		(13,300)	(15,792)	(13,300)		(15,792)
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(72,501)		(67,890)
Net change in unrealized gain (loss)
Reclassification of net (gain) loss realized and included in earnings				3,105		2,916
Valuation adjustment for pension plan amendment	[1]			17,315
Other valuation adjustments for employee benefit plan				(10,782)
Amortization of unrealized net loss on securities transferred to HTM
Income tax expense (benefit)				3,458		1,066
Ending Balance		(66,321)	(66,040)	(66,321)		(66,040)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(4,960)		(178)
Net change in unrealized gain (loss)				(842)		3,611
Reclassification of net (gain) loss realized and included in earnings
Valuation adjustment for pension plan amendment	[1]
Other valuation adjustments for employee benefit plan
Amortization of unrealized net loss on securities transferred to HTM
Income tax expense (benefit)				(309)		1,316
Ending Balance		$ (5,493)	$ 2,117	(5,493)		2,117
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]			$ (3,105)		$ (2,916)

[1] For further discussion on 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).