SCHEDULE I (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 1,157,374
Fair Value	1,362,984
Balance Sheet Value	1,362,984
Debt instruments, trading, at fair value
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	16,448
Fair Value	7,180
Balance Sheet Value	7,180
Equities – listed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,014,426
Fair Value	1,203,672
Balance Sheet Value	1,203,672
Total investments, trading
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,030,874
Fair Value	1,210,852
Balance Sheet Value	1,210,852
Commodities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	101,184
Fair Value	121,502
Balance Sheet Value	121,502
Private and unlisted equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	25,316
Fair Value	30,630
Balance Sheet Value	30,630
Total other investments, at fair value
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	126,500
Fair Value	152,132
Balance Sheet Value	$ 152,132