Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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	$ (1,150)		(1,150)		$ (1,150)
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(24,915)	$ (30,532)	(23,928)	$ (29,869)	(31,336)	$ (34,105)
Other Comprehensive Income (Loss) before Reclassifications	0	0	0	0	7,363	3,777
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(457)	(804)	(1,444)	(1,467)	(1,399)	(1,008)
Accumulated Other Comprehensive Income (Loss), ending balance	(25,372)	(31,336)	(25,372)	(31,336)	(25,372)	(31,336)
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	32,296	28,925	28,463	27,765	30,731	25,769
Other Comprehensive Income (Loss) before Reclassifications	4,768	3,493	14,491	7,459	13,936	12,058
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(1,413)	(1,687)	(7,303)	(4,493)	(9,016)	(7,096)
Accumulated Other Comprehensive Income (Loss), ending balance	35,651	30,731	35,651	30,731	35,651	30,731
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	(11,514)	(11,693)	(11,651)	(11,810)	(11,732)	(11,887)
Other Comprehensive Income (Loss) before Reclassifications	0	0	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	85	(39)	222	78	303	155
Accumulated Other Comprehensive Income (Loss), ending balance	(11,429)	(11,732)	(11,429)	(11,732)	(11,429)	(11,732)
Accumulated Other Comprehensive Income (Loss), Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(4,133)	(13,300)	(7,116)	(13,914)	(12,337)	(20,223)
Other Comprehensive Income (Loss) before Reclassifications	4,768	3,493	14,491	7,459	21,299	15,835
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(1,785)	(2,530)	(8,525)	(5,882)	(10,112)	(7,949)
Accumulated Other Comprehensive Income (Loss), ending balance	$ (1,150)	$ (12,337)	$ (1,150)	$ (12,337)	$ (1,150)	$ (12,337)