Variable Interest Entities (Details) (Variable Interest, Held by Entity [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Variable Interest, Held by Entity [Member]
Carrying amounts of assets and liabilities of the VIE
Operating assets, net of accumulated depreciation	$ 98.4	[1]	$ 104.3	[1]
Nonrecourse debt	$ 35.1		$ 45.2

[1] All operating assets are pledged as collateral on the nonrecourse debt.