Stockholders' Equity (Line Items in Consolidated Income Statements Affected by Amounts Reclassified from Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of defined benefit pension and post-retirement items	[1]	$ 17,315
Employee Benefit Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of defined benefit pension and post-retirement items	[1]	17,315
Amount Reclassified from Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	[2]	(4,579)	$ (3,631)
Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Available for Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on sale of AFS securities	[2]		1,435
Tax effect	[2]		(502)
Net of tax	[2]		933
Amount Reclassified from Accumulated Other Comprehensive Income [Member] | HTM Securities Transferred from AFS [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of unrealized net loss on securities transferred to HTM	[2]	(2,726)	(2,736)
Tax effect	[2]	1,012	1,029
Net of tax	[2]	(1,714)	(1,707)
Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Employee Benefit Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of defined benefit pension and post-retirement items	[2],[3]	(4,144)	(4,395)
Tax effect	[2]	1,491	1,538
Net of tax	[2]	(2,653)	$ (2,857)
Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of loss on terminated cash flash hedges	[2]	(335)
Tax effect	[2]	123
Net of tax	[2]	$ (212)

[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).