Accumulated Other Comprehensive Income (Loss) (Change in AOCI Components net of tax) (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 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)	$ (5,383)	$ 590	(17,046)	8,883
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			0
Balance at End of Period	1,253,327	1,227,893	1,253,327	1,227,893
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(33,252)	(28,599)	(27,715)	(28,892)
Other Comprehensive Income (Loss) Before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	216	146	648	439
Total Other Comprehensive Income (Loss)	216	146	648	439
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			(5,969)
Balance at End of Period	(33,036)	(28,453)	(33,036)	(28,453)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(53,855)	(25,613)	(34,715)	(33,906)
Other Comprehensive Income (Loss) Before Reclassifications	(5,949)	143	(18,953)	7,541
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	566	447	1,907	1,342
Total Other Comprehensive Income (Loss)	(5,383)	590	(17,046)	8,883
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			(7,477)
Balance at End of Period	(59,238)	(25,023)	(59,238)	(25,023)
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	(15,331)	8,668	(1,915)	1,270
Other Comprehensive Income (Loss) Before Reclassifications	(5,949)	143	(18,953)	7,541
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	0	0
Total Other Comprehensive Income (Loss)	(5,949)	143	(18,953)	7,541
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			(412)
Balance at End of Period	(21,280)	8,811	(21,280)	8,811
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	(5,272)	(5,682)	(5,085)	(6,284)
Other Comprehensive Income (Loss) Before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	350	301	1,259	903
Total Other Comprehensive Income (Loss)	350	301	1,259	903
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			(1,096)
Balance at End of Period	$ (4,922)	$ (5,381)	$ (4,922)	$ (5,381)