Investments - Summary of Investment in Private Equity Funds (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	$ 56,184	$ 59,856
Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	10,663	13,812
Private Equity Funds [Member] | ECP II [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	776	833
Private Equity Funds [Member] | Glisco II and III [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	4,127	6,558
Private Equity Funds [Member] | Trilantic IV and V [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	$ 5,760	$ 6,421