Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2016	Mar. 31, 2015
Derivative assets:
Derivative asset, gross	$ 48.6	$ 67.8
Gross amounts offset in the balance sheet	(26.4)	(13.8)
Total derivative assets - net	22.2	54.0
Derivative liabilities:
Derivative liability, gross	(117.1)	(180.8)
Gross amounts offset in the balance sheet	26.4	13.8
Cash collateral pledged	0.1	3.7
Total derivative liabilities - net	(90.6)	(163.3)
Designated as Hedging Instrument
Derivative assets:
Derivative asset, gross	11.6	45.6
Derivative liabilities:
Derivative liability, gross	(9.7)	(12.5)
Designated as Hedging Instrument | Foreign currency contracts
Derivative assets:
Derivative asset, gross	11.6	35.8
Derivative liabilities:
Derivative liability, gross	(5.0)	0.0
Designated as Hedging Instrument | Cross-currency contracts
Derivative assets:
Derivative asset, gross	0.0	9.7
Derivative liabilities:
Derivative liability, gross	(1.3)	0.0
Designated as Hedging Instrument | Interest rate contracts
Derivative assets:
Derivative asset, gross	0.0	0.1
Derivative liabilities:
Derivative liability, gross	(3.4)	(12.5)
Derivatives Subject To PGC and DS Mechanisms | Commodity contracts
Derivative assets:
Derivative asset, gross	1.2	0.0
Derivative liabilities:
Derivative liability, gross	(3.5)	(5.2)
Derivatives Not Designated as Hedging Instruments | Commodity contracts
Derivative assets:
Derivative asset, gross	35.8	22.2
Derivative liabilities:
Derivative liability, gross	$ (103.9)	$ (163.1)