Accumulated Other Comprehensive Income (Loss) - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of accumulated other comprehensive income (loss)
Balance at the beginning of the period	$ (35,218)	$ (29,388)	$ (23,947)
Other comprehensive income (loss), net of tax	19,254	(5,830)	(5,441)
Balance at the end of the period	(15,964)	(35,218)	(29,388)
Unrealized Gain (Loss) on Investments
Components of accumulated other comprehensive income (loss)
Balance at the beginning of the period			(64)
Other comprehensive income (loss), net of tax			64
Retirement plans
Components of accumulated other comprehensive income (loss)
Balance at the beginning of the period	(35,218)	(29,388)	(23,883)
Other comprehensive income (loss), net of tax	19,254	(5,830)	(5,505)
Balance at the end of the period	$ (15,964)	$ (35,218)	$ (29,388)