Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Jan. 01, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	$ 11,058
Accumulated Other Comprehensive Income (Loss), ending balance	(32,321)		$ (32,321)
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					$ 0	$ 0
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					(41,028)	0
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					0	0
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					$ (41,028)	$ 0
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(17,790)	$ (23,928)	(24,457)	$ (30,256)
Other Comprehensive Income (Loss) before Reclassifications	0	0	7,951	7,363
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(685)	(529)	(1,969)	(1,564)
Accumulated Other Comprehensive Income (Loss), ending balance	(18,475)	(24,457)	(18,475)	(24,457)
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	40,190	28,463	32,872	28,394
Other Comprehensive Income (Loss) before Reclassifications	(2,159)	6,165	11,927	11,327
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	404	(1,756)	(6,364)	(6,849)
Accumulated Other Comprehensive Income (Loss), ending balance	(2,593)	32,872	(2,593)	32,872
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	(11,342)	(11,651)	(11,599)	(11,770)
Other Comprehensive Income (Loss) before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	89	52	346	171
Accumulated Other Comprehensive Income (Loss), ending balance	(11,253)	(11,599)	(11,253)	(11,599)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	11,058	(7,116)	(3,184)	(13,632)
Other Comprehensive Income (Loss) before Reclassifications	(2,159)	6,165	19,878	18,690
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(192)	(2,233)	(7,987)	(8,242)
Accumulated Other Comprehensive Income (Loss), ending balance	$ (32,321)	$ (3,184)	$ (32,321)	$ (3,184)