Goodwill and Intangible Assets - Intangible assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Intangible Assets
Gross Value	$ 87,779		$ 87,779		$ 73,889
Accumulated Amortization	(19,102)		(19,102)		(11,153)
Intangible Assets, net	68,677		68,677		$ 62,736
Amortization expense	2,782	$ 1,198	$ 7,949	$ 3,095
Trade name
Intangible Assets
Weighted Average Amortization Period			8 years 18 days		8 years 6 months 22 days
Gross Value	7,306		$ 7,306		$ 6,716
Accumulated Amortization	(2,058)		(2,058)		(1,320)
Intangible Assets, net	5,248		$ 5,248		$ 5,396
Client relationships
Intangible Assets
Weighted Average Amortization Period			8 years 11 months 23 days		8 years 5 months 23 days
Gross Value	44,569		$ 44,569		$ 34,949
Accumulated Amortization	(9,257)		(9,257)		(5,652)
Intangible Assets, net	35,312		$ 35,312		$ 29,297
Non-competition agreement
Intangible Assets
Weighted Average Amortization Period			6 years 11 months 27 days		4 years 11 months 16 days
Gross Value	6,754		$ 6,754		$ 5,404
Accumulated Amortization	(1,558)		(1,558)		(739)
Intangible Assets, net	5,196		$ 5,196		$ 4,665
Developed technology
Intangible Assets
Weighted Average Amortization Period			7 years 7 days		7 years 4 months 17 days
Gross Value	29,091		$ 29,091		$ 26,791
Accumulated Amortization	(6,221)		(6,221)		(3,438)
Intangible Assets, net	22,870		$ 22,870		$ 23,353
Domain name
Intangible Assets
Weighted Average Amortization Period			10 years		10 years
Gross Value	59		$ 59		$ 29
Accumulated Amortization	(8)		(8)		(4)
Intangible Assets, net	$ 51		$ 51		$ 25