Summary of Significant Accounting Policies - Accumulated Other Comprehensive Income (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Changes in accumulated other comprehensive income, net of tax
Balance at December 31, 2016	$ (2,279)
Other comprehensive income and pension reclassification	4,455
Balance at December 31, 2017	2,176
Foreign currency
Changes in accumulated other comprehensive income, net of tax
Balance at December 31, 2016	(1,591)
Other comprehensive income and pension reclassification	4,347
Balance at December 31, 2017	2,756
Defined benefit pension plan
Changes in accumulated other comprehensive income, net of tax
Balance at December 31, 2016	(688)
Other comprehensive income and pension reclassification	108
Balance at December 31, 2017	$ (580)