Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Receivables, allowance for doubtful accounts	$ 3.1	$ 4.6
Restricted cash	260.7	223.2
Property, plant and equipment, at cost	6,275.8	5,642.0
Less: accumulated depreciation	1,505.2	1,343.0
Unamortized discount on recourse debt	74.1	87.5
Preferred stock, shares authorized	1,500,000	1,500,000
Preferred stock, shares unissued	1,500,000	1,500,000
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	81,700,000	81,700,000
Common stock, shares outstanding	81,700,000	81,700,000
Treasury stock, shares	4,300,000	2,600,000
Partially-owned subsidiaries
Restricted cash	77.1	57.8
Property, plant and equipment, at cost	1,887.2	1,661.0
Less: accumulated depreciation	$ 202.1	$ 153.8