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Investments - Additional Information (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Schedule of Investments [Line Items]      
Return of capital $ 0 $ 98,000  
Windfield Holdings      
Schedule of Investments [Line Items]      
Equity method investment, ownership percentage 49.00%    
Carrying value of unconsolidated investment $ 295,800   $ 280,200
Other variable interest entities      
Schedule of Investments [Line Items]      
Carrying value of unconsolidated investment $ 8,600   $ 7,800
Windfield Holdings      
Schedule of Investments [Line Items]      
Return of capital   $ 98,000