Accumulated Other Comprehensive Income (Loss) - Changes in components of AOCI - (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 (Loss)
Balance at the beginning of the period			$ (8,211)
Other comprehensive income (loss), net of tax	$ 1,187	$ 3,005	4,514	$ 10,381
Balance at the end of the period	(3,697)	6,510	(3,697)	6,510
Benefit plans
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(6,078)	(5,889)	(6,195)	(6,015)
Amounts reclassified from accumulated other comprehensive income (loss)	116	126	233	252
Other comprehensive income (loss), net of tax	116	126	233	252
Balance at the end of the period	(5,962)	(5,763)	(5,962)	(5,763)
Unrealized gain and losses on sales of available for sale securities
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	1,458	9,887	(1,708)	2,588
Other comprehensive income (loss) before reclassifications	1,115	2,865	4,282	10,240
Amounts reclassified from accumulated other comprehensive income (loss)	(67)		(68)	(76)
Other comprehensive income (loss), net of tax	1,048	2,865	4,214	10,164
Balance at the end of the period	2,506	12,752	2,506	12,752
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(264)	(493)	(308)	(444)
Other comprehensive income (loss) before reclassifications	11	(28)	(37)	(123)
Amounts reclassified from accumulated other comprehensive income (loss)	12	42	104	88
Other comprehensive income (loss), net of tax	23	14	67	(35)
Balance at the end of the period	(241)	(479)	(241)	(479)
Accumulated Other Comprehensive Income (Loss).
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(4,884)	3,505	(8,211)	(3,871)
Other comprehensive income (loss) before reclassifications	1,126	2,837	4,245	10,117
Amounts reclassified from accumulated other comprehensive income (loss)	61	168	269	264
Other comprehensive income (loss), net of tax	1,187	3,005	4,514	10,381
Balance at the end of the period	$ (3,697)	$ 6,510	$ (3,697)	$ 6,510