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Equity-Based Compensation - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended 54 Months Ended
Apr. 30, 2014
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2014
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Mar. 31, 2015
Nov. 09, 2014
Class A Common Stock                      
Equity-Based Compensation                      
Number of shares of common stock to be issued upon exchange of a partnership unit   1   1     1        
Share value authorized for repurchase                   $ 25,000  
Value of remaining shares authorized for repurchase   $ 20,000   $ 20,000     $ 20,000        
Partnership Units                      
Equity-Based Compensation                      
Compensation expenses   232 $ 469 934 $ 1,668            
Unrecognized compensation expenses   $ 439   $ 439     $ 439        
Weighted average period to recognize unrecognized compensation expense       6 months              
Partnership and Employees                      
Equity-Based Compensation                      
Units held by partners and employees   13,053,465   13,053,465     13,053,465        
Unvested units held by partners and employees   230,026   230,026     230,026        
RSUs                      
Equity-Based Compensation                      
Compensation expenses   $ 29,710 $ 22,057 $ 86,443 $ 69,248            
RSUs | Minimum                      
Equity-Based Compensation                      
Vesting period       4 years              
RSUs | Maximum                      
Equity-Based Compensation                      
Vesting period       5 years              
Restricted Stock and RSUs                      
Equity-Based Compensation                      
Unvested units held by partners and employees   9,283,590 9,451,103 9,283,590 9,451,103   9,283,590 9,357,999 8,504,190    
Unrecognized compensation expenses   $ 141,870   $ 141,870     $ 141,870        
Weighted average period to recognize unrecognized compensation expense       1 year 6 months              
Stock Options                      
Equity-Based Compensation                      
Vesting period           5 years          
Compensation expenses   499 $ 656 $ 1,399 $ 1,996            
Unrecognized compensation expenses   $ 1,150   $ 1,150     $ 1,150        
Grants (in shares) 3,501,881                    
Special dividends paid (in dollars per share)             $ 7.05        
Reduction to exercise price of options outstanding due to special dividend paid (in dollars per share)             7.05        
Exercise price (in dollars per share)   $ 17.95 $ 17.95 $ 17.95 $ 17.95   $ 17.95 $ 17.95 $ 17.95   $ 25.00
Weighted average remaining period for recognition of unrecognized compensation cost       7 months 6 days              
Managing Directors | Partnership Units | Minimum                      
Equity-Based Compensation                      
Vesting period       5 years              
Managing Directors | Partnership Units | Maximum                      
Equity-Based Compensation                      
Vesting period       8 years              
Non-Managing Director Employees | Partnership Units                      
Equity-Based Compensation                      
Vesting period       4 years