Derivative Instruments Offsetting of Derivative Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Offsetting Derivative Assets [Abstract]
Gross amounts recognized	$ 237.8	[1]	$ 149.4	[1]
Gross amounts offset in the balance sheet	0		0
Net amounts presented in the balance sheet	237.8		149.4
Gross amounts not offset in the balance sheet, Financial instruments	(110.2)		(45.7)
Net amount	127.6		103.7
Offsetting Derivative Liabilities [Abstract]
Gross amounts recognized	(110.8)	[1]	(45.7)	[1]
Net amounts presented in the balance sheet	(110.8)		(45.7)
Gross amounts not offset in the balance sheet, Financial instruments	110.2		45.7
Gross amounts not offset in the balance sheet, Cash collateral received/pledged	0.6	[2]	0	[2]
Net amount	0		0
Other Assets
Offsetting Derivative Liabilities [Abstract]
Cash collateral to offset certain individual derivative liability positions	$ 7.2		$ 8.5

[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 $7.2 million and $8.5 million as of December 31, 2013 and 2012, respectively, that exceeds the net liability resulting from the aggregate derivative positions with a corresponding counterparty is excluded.