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Investments - Summary of Investments in Private Equity Funds (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]    
Investment in Private Equity Funds $ 57,323 $ 59,856
Private Equity Funds [Member]    
Schedule of Equity Method Investments [Line Items]    
Investment in Private Equity Funds 9,710 13,812
Private Equity Funds [Member] | ECP II [Member]    
Schedule of Equity Method Investments [Line Items]    
Investment in Private Equity Funds 796 833
Private Equity Funds [Member] | Glisco II, III and IV [Member]    
Schedule of Equity Method Investments [Line Items]    
Investment in Private Equity Funds 3,713 6,558
Private Equity Funds [Member] | Trilantic IV and V [Member]    
Schedule of Equity Method Investments [Line Items]    
Investment in Private Equity Funds $ 5,201 $ 6,421