Derivative Instruments Derivative Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Derivative Asset [Abstract]
Total derivative assets, gross amounts recognized	$ 144.8	[1]	$ 237.8	[1]
Total derivative assets, gross amounts offset in the balance sheet	0		0
Total derivative assets, net amounts presented in the balance sheet	144.8		237.8
Total derivative assets, gross amounts not offset in the balance sheet, financial instruments	(67.2)		(110.2)
Total derivative assets, net amount	77.6		127.6
Offsetting Derivative Liabilities [Abstract]
Total derivative liabilities, gross amounts recognized	(70.0)	[1]	(110.8)	[1]
Total derivative liabilities, gross amounts offset in the balance sheet	9.2		7.2
Total derivative liabilities, net amounts presented in the balance sheet	(70.0)		(110.8)
Total derivative liabilities, gross amounts not offset int he balance sheet, financial instruments	67.2		110.2
Total derivative liabilities, gross amounts not offset in the balance sheet, cash collateral pledged	2.8	[2]	0.6	[2]
Total derivative liability, 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 $9.2 million and $7.2 million as of September 30, 2014 and December 31, 2013, respectively, that exceeds the net liability resulting from the aggregate derivative positions with a corresponding counterparty is excluded.