Investments - Summary of Investment in Private Equity Funds (Details) - Private Equity Funds [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	$ 13,812	$ 21,742
ECP II [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	833	933
Discovery Fund [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	0	7,463
Glisco II [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	5,747	6,897
Glisco III [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	811	529
Trilantic IV [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	305	211
Trilantic V [Member]
Schedule of Equity Method Investments [Line Items]
Investment in Private Equity Funds	$ 6,116	$ 5,709