Fair Value of Financial Instruments - Assets/Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent obligation due selling unitholders of Cogent	$ 17,784	$ 13,763
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	46,224	193,315
Total assets measured at fair value on a recurring basis	46,224	193,315
Contingent obligation due selling unitholders of Cogent	17,784	13,763
Total liabilities measured at fair value on a recurring basis	17,784	13,763
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 and cash equivalents	46,224	193,315
Total assets measured at fair value on a recurring basis	46,224	193,315
Contingent obligation due selling unitholders of Cogent	0	0
Total liabilities measured at fair value on a recurring basis	0	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 and cash equivalents	0	0
Total assets measured at fair value on a recurring basis	0	0
Contingent obligation due selling unitholders of Cogent	17,784	0
Total liabilities measured at fair value on a recurring basis	17,784	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Total assets measured at fair value on a recurring basis	0	0
Contingent obligation due selling unitholders of Cogent	0	13,763
Total liabilities measured at fair value on a recurring basis	$ 0	$ 13,763