Fair Value of Financial Instruments - Assets/Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent obligation due selling unitholders of Cogent	$ 0	$ 18,293
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	52,961	72,193
Total assets measured at fair value on a recurring basis	52,961	72,193
Contingent obligation due selling unitholders of Cogent		18,293
Total liabilities measured at fair value on a recurring basis		18,293
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	52,961	72,193
Total assets measured at fair value on a recurring basis	52,961	72,193
Contingent obligation due selling unitholders of Cogent		0
Total liabilities measured at fair value on a recurring basis		0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Total assets measured at fair value on a recurring basis	0	0
Contingent obligation due selling unitholders of Cogent		18,293
Total liabilities measured at fair value on a recurring basis		18,293
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Total assets measured at fair value on a recurring basis	$ 0	0
Contingent obligation due selling unitholders of Cogent		0
Total liabilities measured at fair value on a recurring basis		$ 0