Accumulated Other Comprehensive Income (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Changes in accumulated other comprehensive income
Amounts reclassified from other comprehensive income	$ 0
Balance at December 31, 2016	265,431
Balance at September 30, 2017	219,152
Unrealized (Loss) Gain on Marketable Securities
Changes in accumulated other comprehensive income
Balance at December 31, 2016	(55)
Other comprehensive gain	47
Balance at September 30, 2017	(8)
Foreign Currency Items
Changes in accumulated other comprehensive income
Balance at December 31, 2016	2,596
Balance at September 30, 2017	2,596
Accumulated Other Comprehensive Income
Changes in accumulated other comprehensive income
Balance at December 31, 2016	2,541
Other comprehensive gain	47
Balance at September 30, 2017	$ 2,588