Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 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	$ (33,746)		(33,746)		$ (33,746)
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	(18,475)	$ (24,457)	(17,790)	$ (23,928)	(24,915)	$ (30,532)
Other Comprehensive Income (Loss) before Reclassifications	0	0	0	0	7,951	7,363
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(649)	(458)	(1,334)	(987)	(2,160)	(1,746)
Accumulated Other Comprehensive Income (Loss), ending balance	(19,124)	(24,915)	(19,124)	(24,915)	(19,124)	(24,915)
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(2,593)	32,872	40,190	28,463	32,296	28,925
Other Comprehensive Income (Loss) before Reclassifications	(1,000)	3,558	(3,159)	9,723	7,369	12,661
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	115	(4,134)	519	(5,890)	(2,115)	(9,290)
Accumulated Other Comprehensive Income (Loss), ending balance	(3,478)	32,296	(3,478)	32,296	(3,478)	32,296
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	(11,253)	(11,599)	(11,342)	(11,651)	(11,514)	(11,693)
Other Comprehensive Income (Loss) before Reclassifications	0	0	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	109	85	198	137	370	179
Accumulated Other Comprehensive Income (Loss), ending balance	(11,144)	(11,514)	(11,144)	(11,514)	(11,144)	(11,514)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(32,321)	(3,184)	11,058	(7,116)	(4,133)	(13,300)
Other Comprehensive Income (Loss) before Reclassifications	(1,000)	3,558	(3,159)	9,723	15,320	20,024
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(425)	(4,507)	(617)	(6,740)	(3,905)	(10,857)
Accumulated Other Comprehensive Income (Loss), ending balance	$ (33,746)	$ (4,133)	$ (33,746)	$ (4,133)	$ (33,746)	$ (4,133)