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Tax Receivable Agreements (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
Apr. 15, 2015
Sep. 30, 2018
May 31, 2018
Feb. 28, 2017
Dec. 31, 2017
Mar. 31, 2019
Dec. 31, 2018
Tax Receivable Agreements [Abstract]              
Payment on applicable cash tax savings (as a percent) 85.00%            
First payment made   $ 12,400   $ 7,000      
Minimum tax receivable agreement obligation over the agreed period $ 4,700            
Maximum tax receivable agreement obligation over the agreed period $ 17,000            
Period over which the obligations are to be settled 15 years            
Deferred tax asset from tax receivable agreement     $ 78,700        
Tax receivable agreement obligations     79,700     $ 214,403 $ 214,403
Issuance of tax receivable agreements in connection with employee exchange     $ 1,000        
Reduction to additional paid-in capital as a result of differences between estimate and tax returns         $ 86,600    
Deferred tax assets           $ 163,600 $ 167,100
Class B              
Class of Stock [Line Items]              
Common stock, par value (in dollars per share)           $ 0.00001 $ 0.00001
Class C              
Class of Stock [Line Items]              
Common stock, par value (in dollars per share)           0.00001 0.00001
Class D              
Class of Stock [Line Items]              
Common stock, par value (in dollars per share)           $ 0.00001 $ 0.00001