Investments - Schedule of Net Investment Earnings (Loss) (Details) - NGP Management - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Schedule of Equity Method Investments [Line Items]
Management fee-related revenues from NGP Management	$ 18.3	$ 18.8
Expenses related to the investment in NGP Management	(2.9)	(2.8)
Amortization of basis differences from the investment in NGP Management	(0.7)	(1.1)
Net investment income	$ 14.7	$ 14.9