Shareholders' Equity Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance	$ (38,449)	$ (22,017)	$ (17,722)
Other comprehensive income (loss) before reclassifications	10,008	(16,007)	(2,798)
Reclassification adjustment for loss on derivative financial instruments	0	0	2,456
Amounts reclassified from accumulated other comprehensive income (loss)	(4,533)	(425)	(3,953)
Ending Balance	(32,974)	(38,449)	(22,017)
Unrealized Gain (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance	(23,047)	(6,499)	5,980
Other comprehensive income (loss) before reclassifications	10,432	(14,891)	(7,717)
Reclassification adjustment for loss on derivative financial instruments			0
Amounts reclassified from accumulated other comprehensive income (loss)	(5,894)	(1,657)	(4,762)
Ending Balance	(18,509)	(23,047)	(6,499)
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance	273	458	1,349
Other comprehensive income (loss) before reclassifications	185	(185)	239
Reclassification adjustment for loss on derivative financial instruments			0
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	(1,130)
Ending Balance	458	273	458
Unrealized Effective Portions of Losses on Forward-Starting Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance	0	(15)	(2,546)
Other comprehensive income (loss) before reclassifications	0	0	0
Reclassification adjustment for loss on derivative financial instruments			2,456
Amounts reclassified from accumulated other comprehensive income (loss)	0	15	75
Ending Balance	0	0	(15)
Unrecognized Pension and Postretirement Plan Income (Cost)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Beginning Balance	(15,675)	(15,961)	(22,505)
Other comprehensive income (loss) before reclassifications	(609)	(931)	4,680
Reclassification adjustment for loss on derivative financial instruments			0
Amounts reclassified from accumulated other comprehensive income (loss)	1,361	1,217	1,864
Ending Balance	$ (14,923)	$ (15,675)	$ (15,961)