Fair Value of Financial Instruments - Quantitative Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Contingent obligation due selling unitholders of Cogent	$ 13,763	$ 15,095
Fair Value, Measurements, Recurring
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Contingent obligation due selling unitholders of Cogent	13,763	15,095
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Contingent obligation due selling unitholders of Cogent	13,763	15,095
Fair Value, Measurements, Recurring | Contingent Consideration Liability | Present value of expected payments | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Contingent obligation due selling unitholders of Cogent	$ 13,763	$ 15,095
Fair Value, Measurements, Recurring | Contingent Consideration Liability | Present value of expected payments | Significant Unobservable Inputs (Level 3) | Range (Weighted Average)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Discount rate	13.00%	13.00%