Accumulated Other Comprehensive Income by Component (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), beginning balance	$ 2,612	$ (66,084)	$ (59,666)	$ (69,943)
Other Comprehensive Income (Loss) before Reclassifications	12,946	5,543	73,337	5,431
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2,499)	875	(612)	4,846
Accumulated Other Comprehensive Income (Loss), ending balance	13,059	(59,666)	13,059	(59,666)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), beginning balance	(21,330)	(75,737)	(75,032)	(77,246)
Other Comprehensive Income (Loss) before Reclassifications	12,147	0	63,599	(1,264)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(205)	705	2,045	3,478
Accumulated Other Comprehensive Income (Loss), ending balance	(9,388)	(75,032)	(9,388)	(75,032)
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), beginning balance	36,240	22,194	27,858	20,049
Other Comprehensive Income (Loss) before Reclassifications	799	5,543	9,738	6,695
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	(2,309)	121	(2,866)	1,114
Accumulated Other Comprehensive Income (Loss), ending balance	34,730	27,858	34,730	27,858
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), beginning balance	(12,298)	(12,541)	(12,492)	(12,746)
Other Comprehensive Income (Loss) before Reclassifications	0	0	0	0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	15	49	209	254
Accumulated Other Comprehensive Income (Loss), ending balance	$ (12,283)	$ (12,492)	$ (12,283)	$ (12,492)