Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Derivative assets:
Derivative asset, gross	$ 58.4	$ 63.3
Gross amounts offset in the balance sheet	(15.6)	(27.5)
Total derivative assets - net	42.8	35.8
Derivative liabilities:
Derivative liability, gross	(177.8)	(314.9)
Gross amounts offset in the balance sheet	15.6	27.5
Cash collateral pledged	5.5	90.5
Total derivative liabilities - net	(156.7)	(196.9)
Designated as Hedging Instrument
Derivative assets:
Derivative asset, gross	27.9	23.1
Designated as Hedging Instrument | Foreign Currency Contracts
Derivative assets:
Derivative asset, gross	25.4	18.7
Designated as Hedging Instrument | Cross Currency Contracts
Derivative assets:
Derivative asset, gross	1.9	4.3
Designated as Hedging Instrument | Interest Rate Contracts
Derivative assets:
Derivative asset, gross	0.6	0.1
Derivative liabilities:
Derivative liability, gross	(9.8)	(17.0)
Derivatives Subject To PGC and DS Mechanisms | Commodity Contracts
Derivative assets:
Derivative asset, gross	0.2	0.2
Derivative liabilities:
Derivative liability, gross	(6.3)	(9.4)
Derivatives Not Designated as Hedging Instruments | Commodity Contracts
Derivative assets:
Derivative asset, gross	30.3	40.0
Derivative liabilities:
Derivative liability, gross	$ (161.7)	$ (288.5)