Derivative Instruments Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative Asset [Abstract]
Derivative Asset, Gross amounts recognized	$ 186.6	$ 149.4
Derivative Asset, Collateral, Obligation to Return Cash, Offset	0	0
Derivative Asset, Net amounts presented in the balance sheet	186.6	149.4
Derivative Assets, Gross amount not offset in the balance sheet, Financial instruments	(76.4)	(45.7)
Net amount	110.2	103.7
Derivative, Collateral, Obligation to Return Cash	8.5	8.5
Offsetting Derivative Liabilities [Abstract]
Derivative Liability, Gross amounts recognized	(76.4)	(45.7)
Derivative Liability, Gross amounts offset in the balance sheet	0	0
Derivative Liability,Net amounts presented in the balance sheet	(76.4)	(45.7)
Derivative Liabilities Gross amount not offset in the balance sheet, Financial instruments	76.4	45.7
Derivative Liabilities Gross amount not offset in the balance sheet, Cash collateral pledged	0	0
Net amount	$ 0	$ 0