Accumulated Other Comprehensive Income (Loss) (Change in AOCI Components net of tax) (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period			$ 1,161,537	$ 1,116,260
Total Other Comprehensive Income	$ 3,253	$ 5,298	8,293	14,133
Balance at End of Period	1,213,757	1,157,219	1,213,757	1,157,219
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(28,746)	(28,720)	(28,892)	(28,861)
Other Comprehensive Income (Loss) Before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	147	141	293	282
Total Other Comprehensive Income	147	141	293	282
Balance at End of Period	(28,599)	(28,579)	(28,599)	(28,579)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(28,866)	(14,722)	(33,906)	(23,557)
Other Comprehensive Income (Loss) Before Reclassifications	2,809	5,011	7,398	13,450
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	444	287	895	683
Total Other Comprehensive Income	3,253	5,298	8,293	14,133
Balance at End of Period	(25,613)	(9,424)	(25,613)	(9,424)
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	5,859	20,998	1,270	12,559
Other Comprehensive Income (Loss) Before Reclassifications	2,809	5,011	7,398	13,450
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0	0	0
Total Other Comprehensive Income	2,809	5,011	7,398	13,450
Balance at End of Period	8,668	26,009	8,668	26,009
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	(5,979)	(7,000)	(6,284)	(7,255)
Other Comprehensive Income (Loss) Before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	297	146	602	401
Total Other Comprehensive Income	297	146	602	401
Balance at End of Period	$ (5,682)	$ (6,854)	$ (5,682)	$ (6,854)