Derivative Instruments Derivative Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivative Asset [Abstract]
Total derivative assets, gross amounts recognized	$ 99.1	$ 157.5
Total derivative assets, gross amounts offset in the balance sheet	0.0	0.0
Derivative Asset	99.1	157.5
Total derivative assets, gross amounts not offset in the balance sheet, financial instruments	(49.6)	(82.6)
Total derivative assets, net amount	49.5	74.9
Offsetting Derivative Liabilities [Abstract]
Total derivative liabilities, gross amounts recognized	(54.3)	(85.6)
Derivative liabilities	(54.3)	(85.6)
Total derivative liabilities, gross amounts not offset int he balance sheet, financial instruments	49.6	82.6
Total derivative liabilities, gross amounts not offset in the balance sheet, cash collateral pledged	4.7	3.0
Total derivative liability, net amount	0.0	0.0
Derivative liability collateral amount exceeding net liability	$ 27.4	$ 13.0