Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of accumulated other comprehensive income (loss)
Balance at the beginning of the period	$ (23,947)	$ (4,716)	$ (28,450)
Other comprehensive income (loss), net of tax	(5,441)	(19,231)	23,734
Balance at the end of the period	(29,388)	(23,947)	(4,716)
Unrealized Gain (Loss) on Investments
Components of accumulated other comprehensive income (loss)
Balance at the beginning of the period	(64)	(60)	(36)
Other comprehensive income (loss), net of tax	64	(4)	(24)
Balance at the end of the period		(64)	(60)
Retirement plans
Components of accumulated other comprehensive income (loss)
Balance at the beginning of the period	(23,883)	(4,656)	(28,414)
Other comprehensive income (loss), net of tax	(5,505)	(19,227)	23,758
Balance at the end of the period	$ (29,388)	$ (23,883)	$ (4,656)