Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value of financial instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Credit valuation adjustments	Dec. 31, 2012 Credit valuation adjustments	Dec. 31, 2012 Credit valuation adjustments Foreign currency forward contracts	Sep. 30, 2013 Recurring Level 1	Dec. 31, 2012 Recurring Level 1	Sep. 30, 2013 Recurring Level 2	Dec. 31, 2012 Recurring Level 2	Sep. 30, 2013 Recurring Total	Dec. 31, 2012 Recurring Total
Assets:
Cash and cash equivalents									$ 170.7	$ 60.2			$ 170.7	$ 60.2
Restricted cash	119.8		119.8		28.6				119.8	28.6			119.8	28.6
Derivative instruments asset											9.8	20.6	9.8	20.6
Total									290.5	88.8	9.8	20.6	300.3	109.4
Liabilities:
Derivative instruments liability											119.9	151.1	119.9	151.1
Total											119.9	151.1	119.9	151.1
Net increase in fair value						13.0	18.4
Pre-tax increase in other comprehensive income						0.6	1.0
Increase in change in fair value of derivatives						12.4	13.8
Foreign exchange gain (loss)	$ 9.1	$ 7.7	$ (12.9)	$ 4.4				$ 3.6