Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	0 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at the beginning of the period	$ (28,450)	$ (4,716)	$ (28,450)	$ (57,518)
Other comprehensive income (loss), net of tax	29,068	(19,231)	23,734	29,068
Balance at the end of the period	(28,450)	(23,947)	(4,716)	(28,450)
Unrealized Gain (Loss) on Investments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at the beginning of the period	(36)	(60)	(36)		(33)
Other comprehensive income (loss), net of tax	(3)	(4)	(24)
Balance at the end of the period		(64)	(60)		(33)
Retirement plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at the beginning of the period	(28,414)	(4,656)	(28,414)		(57,485)
Other comprehensive income (loss), net of tax	29,071	(19,227)	23,758
Balance at the end of the period		$ (23,883)	$ (4,656)		$ (57,485)