Fair Value of Financial Instruments - Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Contingent obligation due selling unitholders of Cogent	$ 15,095	$ 13,647
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Contingent obligation due selling unitholders of Cogent	15,095	13,647
Liabilities	15,095	13,647
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Contingent obligation due selling unitholders of Cogent	0	0
Liabilities	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Contingent obligation due selling unitholders of Cogent	0	0
Liabilities	0	0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Contingent obligation due selling unitholders of Cogent	15,095	13,647
Liabilities	$ 15,095	$ 13,647