Investments - Summary of Investments in Private Equity Funds (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	$ 58,819	$ 58,507
Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	14,382	9,800
ECP II [Member] | Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	774	795
Glisco II, III and IV [Member] | Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	3,763	3,880
Trilantic IV, V and VI [Member] | Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	$ 9,845	$ 5,125