Fair Value Measurements and the Fair Value Option - Schedule of Changes in Level 3 Investments (Details) - Equity Method Investments [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 330.8	$ 236.6	$ 329.4	$ 223.8
Unrealized and realized gains	11.2	10.1	38.9	36.2
Unrealized and realized losses	0.0	(0.6)	(0.8)	(0.1)
Contributions	22.4	10.2	47.5	30.3
Distributions	(21.4)	(29.2)	(95.9)	(64.0)
Other	(0.1)	(0.1)	23.8	0.8
Ending balance	$ 342.9	$ 227.0	$ 342.9	$ 227.0