Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Receivables, allowance for doubtful accounts	$ 28.7	$ 11.8
Unamortized discount on recourse debt	$ 8.5	$ 27.1
Preferred stock, shares authorized	1,500,000	1,500,000
Preferred stock, shares unissued	1,500,000	1,500,000
Common stock, shares authorized	400,000,000.0	400,000,000.0
Common stock, shares issued	150,900,000	152,200,000
Common stock, shares outstanding	150,900,000	152,200,000
Treasury stock, shares	100,000	100,000
Restricted cash	$ 195.2	$ 178.2
Property, plant and equipment, at cost	8,385.2	7,981.0
Less: accumulated depreciation	2,250.5	2,014.2
Partially-owned subsidiaries
Restricted cash	62.9	78.4
Property, plant and equipment, at cost	1,985.9	1,979.8
Less: accumulated depreciation	$ 418.0	$ 364.9