Investments - Summary of Investments in Private Equity Funds (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	$ 54,506	$ 58,507
Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	15,096	9,800
ECP II [Member] | Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	765	795
Glisco II, III and IV [Member] | Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	3,919	3,880
Trilantic IV, V and VI [Member] | Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	$ 10,412	$ 5,125