Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets
Finite-Lived Intangible Assets, Gross	$ 83,830	$ 83,830
Finite-Lived Intangible Assets, Accumulated Amortization	(54,670)	(42,115)
Finite-Lived Intangible Assets, Net	29,160	41,715
Value of Business Acquired
Acquired Finite-Lived Intangible Assets
Finite-Lived Intangible Assets, Gross	42,788	42,788
Finite-Lived Intangible Assets, Accumulated Amortization	(42,788)	(34,335)
Finite-Lived Intangible Assets, Net	0	8,453
Customer Relationships
Acquired Finite-Lived Intangible Assets
Finite-Lived Intangible Assets, Gross	34,661	34,661
Finite-Lived Intangible Assets, Accumulated Amortization	(10,040)	(6,669)
Finite-Lived Intangible Assets, Net	$ 24,621	$ 27,992
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years 7 months	8 years
Trade Names
Acquired Finite-Lived Intangible Assets
Finite-Lived Intangible Assets, Gross	$ 6,381	$ 6,381
Finite-Lived Intangible Assets, Accumulated Amortization	(1,842)	(1,111)
Finite-Lived Intangible Assets, Net	$ 4,539	$ 5,270
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years 7 months	6 years