Derivative Instruments (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair value derivative assets and liabilities classified in condensed balance sheets
Derivative Assets	$ 27.3	$ 33.9
Derivative Liabilities	4.0	56.7
Current
Fair value derivative assets and liabilities classified in condensed balance sheets
Derivative Assets	16.4	23.4
Derivative Liabilities	2.8	37.7
Long-term
Fair value derivative assets and liabilities classified in condensed balance sheets
Derivative Assets	10.9	10.5
Derivative Liabilities	$ 1.2	$ 19.0