Derivative Instruments Derivative Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Derivative Asset [Abstract]
Derivative Asset, Fair Value, Gross Asset	$ 190.7	[1]	$ 149.4	[1]
Derivative Asset, Collateral, Obligation to Return Cash, Offset	0		0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	190.7		149.4
Derivative, Collateral, Obligation to Return Securities	(82.7)		(45.7)
Derivative Asset, Fair Value, Amount Offset Against Collateral	108.0		103.7
Offsetting Derivative Liabilities [Abstract]
Derivative Liability, Fair Value, Gross Liability	(82.7)	[1]	(45.7)	[1]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(82.7)		(45.7)
Derivative, Collateral, Right to Reclaim Securities	82.7		45.7
Derivative, Collateral, Right to Reclaim Cash	0	[2]	0	[2]
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ 0		$ 0

[1] Amounts include all derivative instruments, irrespective of whether there is a legally enforceable master netting arrangement in place.

[2] Cash collateral pledged with derivative counterparties is recorded within other assets on the balance sheets. The Company pledges cash collateral to offset certain individual derivative liability positions with certain counterparties. Cash collateral of $8.5 million and $8.5 million as of September 30, 2013 and December 31, 2012, respectively, that exceeds the net liability resulting from the aggregate derivative positions with a corresponding counterparty is excluded.