Accumulated Other Comprehensive Income (Loss) (Change in AOCI Components net of tax) (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,231,868	$ 1,161,537
Total Other Comprehensive Income (Loss)	(8,905)	5,040
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE	0
Balance at End of Period	1,241,193	1,193,137
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(27,715)	(28,892)
Other Comprehensive Income (Loss) Before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	216	146
Total Other Comprehensive Income (Loss)	216	146
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE	(5,969)
Balance at End of Period	(33,468)	(28,746)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(34,715)	(33,906)
Other Comprehensive Income (Loss) Before Reclassifications	(9,605)	4,589
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	700	451
Total Other Comprehensive Income (Loss)	(8,905)	5,040
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE	(7,477)
Balance at End of Period	(51,097)	(28,866)
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	(1,915)	1,270
Other Comprehensive Income (Loss) Before Reclassifications	(9,605)	4,589
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0
Total Other Comprehensive Income (Loss)	(9,605)	4,589
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE	412
Balance at End of Period	(11,932)	5,859
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	(5,085)	(6,284)
Other Comprehensive Income (Loss) Before Reclassifications	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	484	305
Total Other Comprehensive Income (Loss)	484	305
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE	(1,096)
Balance at End of Period	$ (5,697)	$ (5,979)