Accumulated Other Comprehensive Income Components of Other Comprehensive Income (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 [Roll Forward]
Beginning Balance	$ (34,552)	$ (5,137)	$ (38,449)	$ (22,017)
Other comprehensive income before reclassifications	10,268	12,839	14,541	29,865
Amounts reclassified from accumulated other comprehensive income (loss)	(591)	(13)	(967)	(159)
Ending Balance	(24,875)	7,689	(24,875)	7,689
Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(19,493)	9,911	(23,047)	(6,499)
Other comprehensive income before reclassifications	10,268	12,839	14,541	29,865
Amounts reclassified from accumulated other comprehensive income (loss)	(932)	(49)	(1,651)	(665)
Ending Balance	(10,157)	22,701	(10,157)	22,701
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	273	458	273	458
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	0
Ending Balance	273	458	273	458
Unrealized Effective Portions of Losses on Forward-Starting Interest Rate Swaps
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	0	(11)	0	(15)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	4	0	8
Ending Balance	0	(7)	0	(7)
Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(15,332)	(15,495)	(15,675)	(15,961)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	341	32	684	498
Ending Balance	$ (14,991)	$ (15,463)	$ (14,991)	$ (15,463)