Investments - Schedule of Net Investment Earnings (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Equity Method Investments and Joint Ventures [Abstract]
Management fees	$ 20.0	$ 20.5	$ 37.6	$ 41.2
Performance fees	19.7	9.7	55.3	9.7
Investment income	2.2	5.9	6.2	4.4
Expenses	(9.6)	(3.0)	(35.6)	(6.6)
Amortization of basis differences	(2.2)	(13.8)	(4.3)	(27.6)
Net investment income	$ 30.1	$ 19.3	$ 59.2	$ 21.1