Segment Reporting and Significant Customer Information - Revenue for Individual Countries that Exceed 10% of Total Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Major Customer [Line Items]
Revenues	$ 26,997	$ 24,687	$ 26,411	$ 28,429	$ 26,814	$ 26,823	$ 29,798	$ 30,828	$ 106,524	$ 114,263	$ 123,961
United States
Revenue, Major Customer [Line Items]
Revenues									59,812	62,456	68,441
United Kingdom
Revenue, Major Customer [Line Items]
Revenues									19,113	22,263	25,865
Germany
Revenue, Major Customer [Line Items]
Revenues									12,226	12,576	12,534
Rest of the world
Revenue, Major Customer [Line Items]
Revenues									$ 15,373	$ 16,968	$ 17,121