Accumulated Other Comprehensive Income by Component (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	$ (8,001)	$ 2,612	$ 13,059	$ (59,666)
Other Comprehensive Income (Loss) before Reclassifications	822	12,946	(16,058)	73,337
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2,699)	(2,499)	(6,879)	(612)
Accumulated Other Comprehensive Income (Loss), ending balance	(9,878)	13,059	(9,878)	13,059
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(34,884)	(21,330)	(9,388)	(75,032)
Other Comprehensive Income (Loss) before Reclassifications	0	12,147	(24,775)	63,599
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	256	(205)	(465)	2,045
Accumulated Other Comprehensive Income (Loss), ending balance	(34,628)	(9,388)	(34,628)	(9,388)
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	38,957	36,240	34,730	27,858
Other Comprehensive Income (Loss) before Reclassifications	822	799	8,717	9,738
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2,997)	(2,309)	(6,665)	(2,866)
Accumulated Other Comprehensive Income (Loss), ending balance	36,782	34,730	36,782	34,730
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), beginning balance	(12,074)	(12,298)	(12,283)	(12,492)
Other Comprehensive Income (Loss) before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	42	15	251	209
Accumulated Other Comprehensive Income (Loss), ending balance	$ (12,032)	$ (12,283)	$ (12,032)	$ (12,283)