Derivative Instruments and Hedging Activities - Offsetting Derivative Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative asset, gross	$ 63.3	$ 59.8	[1]
Derivative asset, gross liability	(27.5)	(6.0)
Derivative asset, net	35.8	53.8
Cash collateral (received) pledged	0	0
Derivative asset, net amount recognized in balance sheet	35.8	53.8
Derivative liability, gross	(314.9)	(49.2)	[1]
Derivative liability, gross asset	27.5	6.0
Derivative liability, net	(287.4)	(43.2)
Cash collateral (received) pledged	90.5	0
Derivative liability, net amount recognized in balance sheet	$ (196.9)	$ (43.2)

[1] Certain immaterial amounts have been revised to correct the classification of derivatives.