Investments - Investments in NGP (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Total	$ 1,936.7	$ 1,810.8
Investment in NGP Management
Schedule of Equity Method Investments [Line Items]
Total	374.1	373.5
Principal investments in NGP funds
Schedule of Equity Method Investments [Line Items]
Total	56.9	51.4
NGP Management
Schedule of Equity Method Investments [Line Items]
Total	$ 431.0	$ 424.9