Variable Interest Entities (Details) (Variable Interest, Held by Entity [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 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	$ 101.2	$ 104.3
Nonrecourse debt	$ 40.1	$ 45.2