Derivative Instruments Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Derivative Asset [Abstract]
Derivative Asset, Gross amounts recognized	$ 167.6	[1]	$ 237.8	[1]
Derivative Asset, Gross amounts offset in the balance sheet	0		0
Derivative Asset, Net amounts presented in the balance sheet	167.6		237.8
Derivative Assets, Gross amount not offset in the balance sheet, Financial instruments	(82.3)		(110.2)
Net amount	85.3		127.6
Derivative, Collateral, Obligation to Return Cash	7.5		7.2
Offsetting Derivative Liabilities [Abstract]
Derivative Liability, Gross amounts recognized	(84.6)	[1]	(110.8)	[1]
Derivative Liability, Gross amounts offset in the balance sheet	0		0
Derivative Liability,Net amounts presented in the balance sheet	(84.6)		(110.8)
Derivative Liabilities Gross amount not offset in the balance sheet, Financial instruments	82.3		110.2
Derivative Liabilities Gross amount not offset in the balance sheet, Cash collateral pledged	2.3	[2]	0.6	[2]
Net amount	$ 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 $7.5 million and $7.2 million as of March 31, 2014 and December 31, 2013, respectively, that exceeds the net liability resulting from the aggregate derivative positions with a corresponding counterparty is excluded.