Debt (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Short-Term Debt
Short-term debt	$ 105.2	$ 107.9
Credit facilities	725.0	719.8
Outstanding borrowings related to credit facilities	105.2	107.9
Assets pledged as support for credit facilities	24.9
Weighted-average interest rates on short-term borrowings (as a percent)	3.80%	1.50%
Commercial paper
Short-Term Debt
Short-term debt	50.0	50.0
Credit facilities	579.0	579.0
Outstanding borrowings related to credit facilities	50.0	50.0
Support provided by back-stop facility for commercial paper program (as a percent)	100.00%	100.00%
Other recourse short-term debt
Short-Term Debt
Short-term debt	$ 55.2	$ 57.9