Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance				$ (13,914)
Accumulated Other Comprehensive Income (Loss), ending balance		$ (12,337)		(12,337)		$ (12,337)
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance		(30,532)	$ (34,331)	(29,869)	$ (34,884)	(34,105)	$ (9,818)
Other Comprehensive Income (Loss) before Reclassifications		0	0	0	0	3,777	(24,775)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	[1]	(804)	226	(1,467)	779	(1,008)	488
Accumulated Other Comprehensive Income (Loss), ending balance		(31,336)	(34,105)	(31,336)	(34,105)	(31,336)	(34,105)
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance		28,925	35,726	27,765	38,957	25,769	39,525
Other Comprehensive Income (Loss) before Reclassifications		3,493	(6,485)	7,459	(6,854)	12,058	(4,537)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		(1,687)	(3,472)	(4,493)	(6,334)	(7,096)	(9,219)
Accumulated Other Comprehensive Income (Loss), ending balance		30,731	25,769	30,731	25,769	30,731	25,769
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		(11,693)	(11,959)	(11,810)	(12,074)	(11,887)	(12,146)
Other Comprehensive Income (Loss) before Reclassifications		0	0	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		(39)	72	78	187	155	259
Accumulated Other Comprehensive Income (Loss), ending balance		(11,732)	(11,887)	(11,732)	(11,887)	(11,732)	(11,887)
Accumulated Other Comprehensive Income (Loss), Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance		(13,300)	(10,564)	(13,914)	(8,001)	(20,223)	17,561
Other Comprehensive Income (Loss) before Reclassifications		3,493	(6,485)	7,459	(6,854)	15,835	(29,312)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		(2,530)	(3,174)	(5,882)	(5,368)	(7,949)	(8,472)
Accumulated Other Comprehensive Income (Loss), ending balance		$ (12,337)	$ (20,223)	$ (12,337)	$ (20,223)	$ (12,337)	$ (20,223)

[1] These items are included in the computation of net periodic benefit cost. See Note I, Employee Benefits, for additional information.