Fair Value Measurements - Schedule of investments, categorized by the level of the fair value hierarchy (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$ 500		$ 500
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	$ 0	0	$ 0
Assets
Private common equity securities	3,000		3,000
Total equities	3,500		3,500
U.S. Treasury securities	197,664		197,664		$ 197,312
Sovereign debt	22,381		22,381		42,328
Total debt securities	220,045		220,045		239,640
Total assets	223,545		223,545
Investments in funds valued at NAV	818,600		818,600		1,284,088
Assets Including Alternative Investments, Fair Value Disclosure	1,042,145		1,042,145		1,523,728
Liabilities
Derivative liabilities (embedded)	37		37		22
Total liabilities	37		37		22
Debt Securities
Liabilities
Trading securities, change in unrealized holding gains (loss)	4,400	(1,600)	6,800	(6,900)
Equity securities
Liabilities
Trading securities, change in unrealized holding gains (loss)	0	$ 0	0	$ 0
Quoted prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0		0
Assets
Private common equity securities	0		0
Total equities	0		0
U.S. Treasury securities	0		0		0
Sovereign debt	0		0		0
Total debt securities	0		0		0
Total assets	0		0
Liabilities
Derivative liabilities (embedded)	0		0		0
Total liabilities	0		0		0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0		0
Assets
Private common equity securities	0		0
Total equities	0		0
U.S. Treasury securities	197,664		197,664		197,312
Sovereign debt	22,381		22,381		42,328
Total debt securities	220,045		220,045		239,640
Total assets	220,045		220,045
Liabilities
Derivative liabilities (embedded)	0		0		0
Total liabilities	0		0		0
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	500		500
Assets
Private common equity securities	3,000		3,000
Total equities	3,500		3,500
U.S. Treasury securities	0		0		0
Sovereign debt	0		0		0
Total debt securities	0		0		0
Total assets	3,500		3,500
Liabilities
Derivative liabilities (embedded)	37		37		22
Total liabilities	$ 37		$ 37		$ 22