Derivative Instruments Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative Asset [Abstract]
Derivative Asset, Fair Value, Gross Asset	$ 199.3	$ 149.4
Derivative Asset, Collateral, Obligation to Return Cash, Offset	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	199.3	149.4
Derivative, Collateral, Obligation to Return Securities	(77.4)	(45.7)
Net amount	121.9	103.7
Derivative, Collateral, Obligation to Return Cash	8.5	8.5
Offsetting Derivative Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(77.4)	(45.7)
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(77.4)	(45.7)
Derivative, Collateral, Right to Reclaim Securities	77.4	45.7
Derivative, Collateral, Right to Reclaim Cash	0	0
Net amount	$ 0	$ 0