Investments - Schedule of Net Investment Earnings (Loss) (Details) - NGP Management - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule of Equity Method Investments [Line Items]
Management fee-related revenues from NGP Management	$ 18.6	$ 23.6	$ 56.8	$ 75.2
Expenses related to the investment in NGP Management	(2.9)	(2.7)	(8.6)	(7.9)
Amortization of basis differences from the investment in NGP Management	(1.1)	(1.5)	(3.2)	(4.3)
Net investment income	$ 14.6	$ 19.4	$ 45.0	$ 63.0