Accumulated Other Comprehensive Income (Loss) - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of accumulated other comprehensive income (loss)
Balance at the beginning of the period	$ (29,388)	$ (23,947)	$ (4,716)
Other comprehensive income (loss)	(5,830)	(5,441)	(19,231)
Balance at the end of the period	(35,218)	(29,388)	(23,947)
Unrealized Gain (Loss) on Investments
Components of accumulated other comprehensive income (loss)
Balance at the beginning of the period		(64)	(60)
Other comprehensive income (loss)		64	(4)
Balance at the end of the period			(64)
Retirement plans
Components of accumulated other comprehensive income (loss)
Balance at the beginning of the period	(29,388)	(23,883)	(4,656)
Other comprehensive income (loss)	(5,830)	(5,505)	(19,227)
Balance at the end of the period	$ (35,218)	$ (29,388)	$ (23,883)