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Compensation and Benefits - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Compensation Arrangements [Line Items]      
Delayed Exchange Class B units issued $ 4 $ 25  
Deferred compensation liability 1,200   $ 918
Unrecorded compensation expense $ 32,800   $ 32,600
Common Stock | Common Class B      
Compensation Arrangements [Line Items]      
Delayed Exchange Class B units issued (in shares) 547 3,888  
Delayed Exchange Class B units issued $ 100 $ 100  
Phantom Shares of Class A common stock | Director      
Compensation Arrangements [Line Items]      
Deferred compensation percentage of compensation directors elected 100.00%    
Shares outstanding (in shares) 383,912   336,016
Distributions made under Director Plan (in shares) 0 0  
Equity Incentive Plan 2006      
Compensation Arrangements [Line Items]      
Cash compensation paid in the form of equity     $ 4,200
Bonus plan      
Compensation Arrangements [Line Items]      
Vesting period for units (in years) 4 years    
Contingent Options to Purchase Shares of Class A Common Stock | 2007 Equity Incentive Plan      
Compensation Arrangements [Line Items]      
Contingently vesting options (in shares) 0 0