Shareholders' Equity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Before-Tax Amount
Unrealized gain on securities	$ 16,051	$ (22,907)	$ (11,872)
Reclassification adjustment for securities gains included in net income (1)	(9,071)	(2,550)	(9,066)
Reclassification adjustment for loss on derivative financial instruments included in net income (2)			3,778
Non-credit related unrealized loss on other-than-temporarily impaired debt securities	285	(285)	368
Amortization of unrealized loss on derivative financial instruments (2)		25	115
Unrecognized pension and postretirement cost	(937)	(1,432)	7,200
Amortization of net unrecognized pension and postretirement income (2)	2,092	1,869	2,869
Total Other Comprehensive Income	8,420	(25,280)	(6,608)
Tax Effect
Unrealized gain on securities	(5,619)	8,016	4,155
Reclassification adjustment for securities gains included in net income (1)	3,177	893	3,174
Reclassification adjustment for loss on derivative financial instruments included in net income (2)			(1,322)
Non-credit related unrealized loss on other-than-temporarily impaired debt securities	(100)	100	(129)
Amortization of unrealized loss on derivative financial instruments (2)		(9)	(40)
Unrecognized pension and postretirement cost	328	501	(2,520)
Amortization of net unrecognized pension and postretirement income (2)	(731)	(653)	(1,005)
Total Other Comprehensive Income	(2,945)	8,848	2,313
Net of Tax Amount
Unrealized gain on securities	10,432	(14,891)	(7,717)
Reclassification adjustment for securities gains included in net income (1)	(5,894)	(1,657)	(5,892)
Reclassification adjustment for loss on derivative financial instruments included in net income (2)	0	0	2,456
Non-credit related unrealized loss on other-than-temporarily impaired debt securities	185	(185)	239
Amortization of unrealized loss on derivative financial instruments (2)	0	16	75
Unrecognized pension and postretirement cost	(609)	(931)	4,680
Amortization of net unrecognized pension and postretirement income (2)	1,361	1,216	1,864
Total Other Comprehensive Income	$ 5,475	$ (16,432)	$ (4,295)