Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 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		(13,632)		$ (13,632)
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance		(29,869)	$ (34,884)	(34,628)	$ (9,388)
Other Comprehensive Income (Loss) before Reclassifications		0	0	3,777	(24,775)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	[1]	(387)	256	595	(465)
Accumulated Other Comprehensive Income (Loss), ending balance		(30,256)	(34,628)	(30,256)	(34,628)
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance		27,765	38,957	36,782	34,730
Other Comprehensive Income (Loss) before Reclassifications		1,742	822	(1,335)	8,717
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		(1,113)	(2,997)	(7,053)	(6,665)
Accumulated Other Comprehensive Income (Loss), ending balance		28,394	36,782	28,394	36,782
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		(11,810)	(12,074)	(12,032)	(12,283)
Other Comprehensive Income (Loss) before Reclassifications		0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		40	42	262	251
Accumulated Other Comprehensive Income (Loss), ending balance		(11,770)	(12,032)	(11,770)	(12,032)
Accumulated Other Comprehensive Income (Loss), Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance		(13,914)	(8,001)	(9,878)	13,059
Other Comprehensive Income (Loss) before Reclassifications		1,742	822	2,442	(16,058)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)		(1,460)	(2,699)	(6,196)	(6,879)
Accumulated Other Comprehensive Income (Loss), ending balance		$ (13,632)	$ (9,878)	$ (13,632)	$ (9,878)

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