Summary of Significant Accounting Policies Derivative instruments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives	$ 63.2	$ 117.7	$ 121.7
Accumulated Other Comprehensive Income Loss, Cumulative Changes In Gain Loss From Cash Flow Hedges Effect, Before Tax	123.5	395.9
Unrealized gains on derivative contracts, net of tax	$ 77.6	$ 248.6