Fair Value Measurements - Schedule of investments, categorized by the level of the fair value hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Equity securities	$ 1,000
Private common equity securities	3,000
Total equities	4,000
U.S. Treasury securities	101,186	$ 197,312
Sovereign debt	23,885	42,328
Total debt securities	125,071	239,640
Total assets	129,071
Investments in funds valued at NAV	860,630	1,284,088
Assets Including Alternative Investments, Fair Value Disclosure	989,701	1,523,728
Liabilities
Total liabilities	31	22
Derivative liabilities (embedded)	31	22
Quoted prices in active markets (Level 1)
Assets
Equity securities	0
Private common equity securities	0
Total equities	0
U.S. Treasury securities	0	0
Sovereign debt	0	0
Total debt securities	0	0
Total assets	0
Liabilities
Total liabilities	0	0
Derivative liabilities (embedded)	0	0
Significant other observable inputs (Level 2)
Assets
Equity securities	0
Private common equity securities	0
Total equities	0
U.S. Treasury securities	101,186	197,312
Sovereign debt	23,885	42,328
Total debt securities	125,071	239,640
Total assets	125,071
Liabilities
Total liabilities	0	0
Derivative liabilities (embedded)	0	0
Significant unobservable inputs (Level 3)
Assets
Equity securities	1,000
Private common equity securities	3,000
Total equities	4,000
U.S. Treasury securities	0	0
Sovereign debt	0	0
Total debt securities	0	0
Total assets	4,000
Liabilities
Total liabilities	31	22
Derivative liabilities (embedded)	$ 31	$ 22