Accumulated Other Comprehensive Income Components of Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance			$ (32,974)
Ending Balance	$ (73,258)		(73,258)
Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(50,056)	$ (19,493)	(18,509)	$ (23,047)
Other comprehensive loss before reclassifications	(6,631)	10,268	(34,275)	14,541
Amounts reclassified from accumulated other comprehensive income (loss)	(3)	(932)	(19)	(1,651)
Ending Balance	(56,690)	(10,157)	(56,690)	(10,157)
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	682	273	458	273
Other comprehensive loss before reclassifications	8	0	232	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	0
Ending Balance	690	273	690	273
Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(17,798)	(15,332)	(14,923)	(15,675)
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	540	341	879	684
Ending Balance	(17,258)	(14,991)	(17,258)	(14,991)
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(67,172)	(34,552)	(32,974)	(38,449)
Other comprehensive loss before reclassifications	(6,623)	10,268	(34,043)	14,541
Amounts reclassified from accumulated other comprehensive income (loss)	537	(591)	860	(967)
Ending Balance	$ (73,258)	$ (24,875)	(73,258)	$ (24,875)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)			3,887
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)			0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)			3,214
Reclassification out of Accumulated Other Comprehensive Income [Member] | Total
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income (loss)			$ 7,101