Stockholders' Equity (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ (134,402)	$ (120,532)
Net change in unrealized gain (loss)		(62,244)	1,184
Reclassification of net (gain) loss realized and included in earnings		1,796	1,387
Amortization of unrealized net loss on securities transferred to HTM		755	650
Income tax expense (benefit)		(13,546)	1,201
Ending Balance		(180,549)	(118,512)
Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(29,512)	(28,679)
Net change in unrealized gain (loss)		(55,114)	2,319
Income tax expense (benefit)		(12,508)	843
Ending Balance		(72,118)	(27,203)
HTM 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		755	650
Income tax expense (benefit)		171	266
Ending Balance		(14,001)	(14,008)
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(79,078)	(72,501)
Reclassification of net (gain) loss realized and included in earnings		1,177	1,387
Income tax expense (benefit)		267	504
Ending Balance		(78,168)	(71,618)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(11,227)	(4,960)
Net change in unrealized gain (loss)		(7,130)	(1,135)
Reclassification of net (gain) loss realized and included in earnings		619
Income tax expense (benefit)		(1,476)	(412)
Ending Balance		(16,262)	(5,683)
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]	$ (1,177)	$ (1,387)

[1] 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).

[2] Three Months EndedAmount reclassified from AOCI (a) March 31,Affected line item on(in thousands)20182017the statement of incomeAmortization of unrealized net loss on securities transferred to HTM (755) (650)Interest incomeTax effect 171 266Income taxesNet of tax (584) (384)Net incomeAmortization of defined benefit pension and post-retirement items (1,177) (1,387)Other noninterest expense (b)Tax effect 267 504Income taxesNet of tax (910) (883)Net incomeReclassification of unrealized gain on cash flow hedges 336 -Interest incomeTax effect (76) -Income taxesNet of tax 260Net incomeAmortization of loss on terminated cash flow hedges (954) -Interest incomeTax effect 216 -Income taxesNet of tax (738) -Net incomeTotal reclassifications, net of tax$ (1,972)$ (1,267)Net incomeAmounts in parenthesis indicate reduction in net income.