Investments - Strategic Investment in NGP (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Total	$ 2,443.6	$ 1,765.8
Investment in NGP Management
Schedule of Equity Method Investments [Line Items]
Total	383.6	394.6
Investments in NGP general partners - accrued performance allocations
Schedule of Equity Method Investments [Line Items]
Total	0.0	151.0
Principal investments in NGP funds
Schedule of Equity Method Investments [Line Items]
Total	67.9	77.6
Total investments in NGP
Schedule of Equity Method Investments [Line Items]
Total	$ 451.5	$ 623.2