Shareholders' Equity (AOCI rollforward) (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,161,537	$ 1,116,260	$ 1,055,086
Other Comprehensive Income (Loss)	(809)	(10,349)	3,129
Balance at End of Period	1,231,868	1,161,537	1,116,260
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(28,892)	(28,861)	(34,115)
Other Comprehensive Income (Loss) Before Reclassification	535	(577)	4,466
Amounts Reclassified from Accumulated Other Comprehensive Income	642	546	788
Other Comprehensive Income (Loss)	1,177	(31)	5,254
Balance at End of Period	(27,715)	(28,892)	(28,861)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(33,906)	(23,557)	(26,686)
Other Comprehensive Income (Loss) Before Reclassification	(2,650)	(11,866)	1,156
Amounts Reclassified from Accumulated Other Comprehensive Income	1,841	1,517	1,973
Other Comprehensive Income (Loss)	(809)	(10,349)	3,129
Balance at End of Period	(34,715)	(33,906)	(23,557)
Available-for-sale Securities | Unrealized Gains and Losses on Net Investment Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	1,270	12,559	15,984
Other Comprehensive Income (Loss) Before Reclassification	(3,185)	(11,289)	(3,310)
Amounts Reclassified from Accumulated Other Comprehensive Income	0	0	(115)
Other Comprehensive Income (Loss)	(3,185)	(11,289)	(3,425)
Balance at End of Period	(1,915)	1,270	12,559
Held-to-maturity Securities | Unrealized Gains and Losses on Net Investment Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(6,284)	(7,255)	(8,555)
Other Comprehensive Income (Loss) Before Reclassification	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income	1,199	971	1,300
Other Comprehensive Income (Loss)	1,199	971	1,300
Balance at End of Period	$ (5,085)	$ (6,284)	$ (7,255)