Other Comprehensive Income (Loss) (Schedule of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$ 364,571
Other Comprehensive Income (Loss) Before Reclassification	(9,005)	$ 3,998
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(213)	0
Net Current Period Other Comprehensive Income (Loss)	(9,218)	3,998
Ending Balance	364,005
Unrealized Gains and Losses on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(2,335)	(6,312)
Other Comprehensive Income (Loss) Before Reclassification	(9,005)	3,998
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(213)	0
Net Current Period Other Comprehensive Income (Loss)	(9,218)	3,998
Ending Balance	(11,553)	(2,314)
Postretirement Benefit Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(285)	(92)
Other Comprehensive Income (Loss) Before Reclassification	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0	0
Net Current Period Other Comprehensive Income (Loss)	0	0
Ending Balance	(285)	(92)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(2,620)	(6,404)
Ending Balance	$ (11,838)	$ (2,406)