Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains on investments									$ 305	$ 550	$ 593
Tax effect									(109)	(8,310)	(19,578)
Amounts reclassified from AOCI, net of tax	(7,255)	30,755	7,978	9,312	(4,403)	8,871	6,516	8,453	41,178	19,320	3,553
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains on Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains on investments									90	99
Tax effect									(34)	(38)
Amounts reclassified from AOCI, net of tax									56	61
Reclassification out of Accumulated Other Comprehensive Income | Unrecognized Net Actuarial Loss and Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax effect									(1,235)	(159)
Recognized net actuarial loss									(3,821)	(1,349)
Amortization of prior service credit									565	902
Amounts reclassified from AOCI, net of tax									$ (2,021)	$ (288)