Accumulated Other Comprehensive Income Components of Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		$ (32,974)
Reclassification of stranded tax effects	[1]	0
Ending Balance		(67,172)
Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		(18,509)	$ (23,047)
Other comprehensive loss before reclassifications		(27,644)	4,273
Amounts reclassified from accumulated other comprehensive income (loss)		(16)	(719)
Reclassification of stranded tax effects		(3,887)
Ending Balance		(50,056)	(19,493)
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		458	273
Other comprehensive loss before reclassifications		224	0
Amounts reclassified from accumulated other comprehensive income (loss)		0	0
Reclassification of stranded tax effects		0
Ending Balance		682	273
Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		(14,923)	(15,675)
Other comprehensive loss before reclassifications		0	0
Amounts reclassified from accumulated other comprehensive income (loss)		339	343
Reclassification of stranded tax effects		(3,214)
Ending Balance		(17,798)	(15,332)
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		(32,974)	(38,449)
Other comprehensive loss before reclassifications		(27,420)	4,273
Amounts reclassified from accumulated other comprehensive income (loss)		323	(376)
Reclassification of stranded tax effects	[1]	(7,101)
Ending Balance		$ (67,172)	$ (34,552)

[1] Result of adoption of ASU 2018-02, See Note 1 to Consolidated Financial Statements for further details.