Investments - Investments Carrying Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Carrying Value	$ 199.7	$ 377.4
Cost method, private equity and other investments
Investment [Line Items]
Carrying Value	50.9	16.8
Available-for-sale securities
Investment [Line Items]
Carrying Value	11.8	180.5
Available-for-sale securities | State Street Corporation
Investment [Line Items]
Carrying Value	0.0	169.6
Available-for-sale securities | Other available-for-sale securities
Investment [Line Items]
Carrying Value	11.8	10.9
Other investments
Investment [Line Items]
Carrying Value	187.9	196.9
Other investments | Trading securities
Investment [Line Items]
Carrying Value	33.6	7.9
Other investments | Seed Capital Investments, at fair value
Investment [Line Items]
Carrying Value	12.1	61.0
Other investments | Cost method, private equity and other investments
Investment [Line Items]
Carrying Value	$ 142.2	$ 128.0