Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	$ (7,116)
Accumulated Other Comprehensive Income (Loss), ending balance	11,058	$ (7,116)
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(23,928)	(29,869)	$ (34,884)
Other Comprehensive Income (Loss) before Reclassifications	7,951	7,363	3,777
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(1,813)	(1,422)	1,238
Accumulated Other Comprehensive Income (Loss), ending balance	(17,790)	(23,928)	(29,869)
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	28,463	27,765	38,957
Other Comprehensive Income (Loss) before Reclassifications	20,251	6,904	(2,255)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(8,524)	(6,206)	(8,937)
Accumulated Other Comprehensive Income (Loss), ending balance	40,190	28,463	27,765
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(11,651)	(11,810)	(12,074)
Other Comprehensive Income (Loss) before Reclassifications	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	309	159	264
Accumulated Other Comprehensive Income (Loss), ending balance	(11,342)	(11,651)	(11,810)
Accumulated Other Comprehensive Income (Loss), Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(7,116)	(13,914)	(8,001)
Other Comprehensive Income (Loss) before Reclassifications	28,202	14,267	1,522
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(10,028)	(7,469)	(7,435)
Accumulated Other Comprehensive Income (Loss), ending balance	$ 11,058	$ (7,116)	$ (13,914)