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Tax Receivable Agreement Liability - Schedule of Tax Agreement Liability (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2018
Tax Receivable Agreement [Roll Forward]        
Payable pursuant to tax receivable agreement $ 55,046   $ 82,291 $ 82,291
Additions (reductions) to tax receivable agreement:        
Adjustment for change in estimated tax rate 0 $ 30,317 $ 2,615  
Payments under tax receivable agreement 0     (4,293)
Payable pursuant to tax receivable agreement 57,599     55,046
Less current portion under tax receivable agreement (3,932)     (3,932)
Payable pursuant to tax receivable agreement, less current portion 53,667     51,114
Exchange of LLC Units for Class A Shares        
Additions (reductions) to tax receivable agreement:        
Exchange of LLC Units for Class A Common Stock 2,553     1,685
Adjustment for change in estimated tax rate $ 0     $ (24,637)