Investments - Strategic Investment in NGP - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Performance allocations, percentage			47.50%
Basis difference	$ 5,300,000		$ 5,300,000		$ 8,500,000
Amortization period of basis difference			10 years
NGP Management
Schedule of Equity Method Investments [Line Items]
Net investment earnings related to performance allocations	0	$ (81,600,000)	$ 0	$ (111,500,000)
Net investment earnings related to principal investment income	$ 900,000	$ (3,800,000)	$ (14,000,000.0)	$ (7,400,000)
Management Fee Related Revenues
Schedule of Equity Method Investments [Line Items]
Management fee-related revenues, allocation percentage			55.00%
Minimum | NGP Management
Schedule of Equity Method Investments [Line Items]
Management fee, percentage of commitments			1.00%
Management fee, percentage of invested capital			0.60%
Maximum | NGP Management
Schedule of Equity Method Investments [Line Items]
Management fee, percentage of commitments			2.00%
Management fee, percentage of invested capital			2.00%