Investments AFS (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 1,295.3	$ 501.6
Amount of short-term investments	1,295.3	394.1
Amount of long-term investments	0.0	107.5
Proceeds for sale of available-for-sale securities		470.2	$ 1,501.5
Realized gain on available-for-sale investments			$ 13.7
Impairment of Available-for-sale Securities	15.5
Corporate bonds and debt
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	542.9	207.8
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 2.8	$ 1.6