Investments - Investments in NGP (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Total	$ 2,039.3	$ 1,810.8
Investment in NGP Management
Schedule of Equity Method Investments [Line Items]
Total	372.5	373.5
Investments in NGP general partners - accrued performance allocations
Schedule of Equity Method Investments [Line Items]
Total	2.9	0.0
Principal investments in NGP funds
Schedule of Equity Method Investments [Line Items]
Total	62.3	51.4
Total investments in NGP
Schedule of Equity Method Investments [Line Items]
Total	$ 437.7	$ 424.9