Fair Value of Financial Instruments - Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Liabilities
Contingent obligation due selling unitholders of Cogent	$ 13,763	$ 15,095
Fair Value, Measurements, Recurring
Assets
Cash and cash equivalents	183,369
Assets, Fair Value Disclosure, Recurring	183,369
Liabilities
Contingent obligation due selling unitholders of Cogent	13,763	15,095
Liabilities	13,763	15,095
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Assets
Cash and cash equivalents	183,369
Assets, Fair Value Disclosure, Recurring	183,369
Liabilities
Contingent obligation due selling unitholders of Cogent	0	0
Liabilities	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Assets
Cash and cash equivalents	0
Assets, Fair Value Disclosure, Recurring	0
Liabilities
Contingent obligation due selling unitholders of Cogent	0	0
Liabilities	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Assets
Cash and cash equivalents	0
Assets, Fair Value Disclosure, Recurring	0
Liabilities
Contingent obligation due selling unitholders of Cogent	13,763	15,095
Liabilities	$ 13,763	$ 15,095