Accumulated Other Comprehensive Income (Loss) - Changes in components of AOCI - (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	$ (8,211)
Other comprehensive income (loss), net of tax	3,327	$ 7,376
Balance at the end of the period	(4,884)	3,505
Benefit plans
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(6,195)	(6,015)
Amounts reclassified from accumulated other comprehensive income (loss)	117	126
Other comprehensive income (loss), net of tax	117	126
Balance at the end of the period	(6,078)	(5,889)
Unrealized gain and losses on sales of available for sale securities
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(1,708)	2,588
Other comprehensive income (loss) before reclassifications	3,166	7,375
Amounts reclassified from accumulated other comprehensive income (loss)		(76)
Other comprehensive income (loss), net of tax	3,166	7,299
Balance at the end of the period	1,458	9,887
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(308)	(444)
Other comprehensive income (loss) before reclassifications	(24)	(94)
Amounts reclassified from accumulated other comprehensive income (loss)	68	45
Other comprehensive income (loss), net of tax	44	(49)
Balance at the end of the period	(264)	(493)
Accumulated Other Comprehensive Income (Loss).
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(8,211)	(3,871)
Other comprehensive income (loss) before reclassifications	3,142	7,281
Amounts reclassified from accumulated other comprehensive income (loss)	185	95
Other comprehensive income (loss), net of tax	3,327	7,376
Balance at the end of the period	$ (4,884)	$ 3,505