Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in accumulated other comprehensive income [Roll Forward]
Balance	$ 391,979	$ 374,153	$ 349,432
Other comprehensive income before reclassifications	(4,962)	(5,091)
Amounts reclassified from accumulated other comprehensive income	408	1,269
Other comprehensive income, net	(4,554)	(3,822)	(4,438)
Balance	441,028	391,979	374,153
Accumulated Other Comprehensive Income (Loss) [Member]
Changes in accumulated other comprehensive income [Roll Forward]
Balance	(6,474)	(2,652)	1,786
Other comprehensive income, net	(4,554)	(3,822)	(4,438)
Balance	(11,028)	(6,474)	(2,652)
Foreign Currency Translation Adjustments [Member]
Changes in accumulated other comprehensive income [Roll Forward]
Balance	(5,958)	(219)
Other comprehensive income before reclassifications	(5,294)	(5,739)
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income, net	(5,294)	(5,739)
Balance	(11,252)	(5,958)	(219)
Unrecognized Postretirement Benefit Costs (Credit) [Member]
Changes in accumulated other comprehensive income [Roll Forward]
Balance	(516)	(2,433)
Other comprehensive income before reclassifications	332	648
Amounts reclassified from accumulated other comprehensive income	408	1,269
Other comprehensive income, net	740	1,917
Balance	$ 224	$ (516)	$ (2,433)