Business Segments (Details 3) - 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
Segment Reporting Information [Line Items]
Revenue	$ 131,503	$ 124,097	$ 131,161	$ 122,447	$ 127,283	$ 121,978	$ 125,542	$ 115,756	$ 509,208	$ 490,559	$ 490,280
United States
Segment Reporting Information [Line Items]
Revenue									350,632	339,329	341,581
United Kingdom
Segment Reporting Information [Line Items]
Revenue									100,466	93,017	98,991
Other Countries
Segment Reporting Information [Line Items]
Revenue									$ 58,110	$ 58,213	$ 49,708