Fair Value of Financial Instruments - Narrative (Details) $ in Thousands		3 Months Ended		9 Months Ended
	Apr. 01, 2015 USD ($) shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Revenue target	$ 80,000
Distribution period ending on the second anniversary of the closing (years)	2 years
Distribution period ending on the fourth anniversary of the closing (years)	2 years
Distribution period ending On March 31 2017 (years)	2 years
Distribution period ending On March 31 2019 (years)	2 years
Contingent obligation due selling unitholders of Cogent		$ 17,784		$ 17,784		$ 13,763
Increase (decrease) in fair value contingent consideration				4,022	$ (5,639)
Cogent Partners, LP
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maximum contingent consideration	$ 18,900
Conditional consideration (shares) | shares	334,048
Contingent obligation due selling unitholders of Cogent	$ 13,100	17,800		17,800
Increase (decrease) in fair value contingent consideration		$ 400	$ 1,874	$ 4,000	$ (5,639)
Measurement Input, Discount Rate | Cogent Partners, LP
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument, measurement input		0.126		0.126