Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Receivables, allowance for doubtful accounts	$ 19.9	$ 20.1
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	400,000,000
Common stock, shares issued	133,300,000	150,900,000
Common stock, shares outstanding	133,300,000	150,900,000
Treasury stock, shares	100,000	100,000
Restricted cash	$ 171.6	$ 195.2
Property, plant and equipment, at cost	8,253.4	7,330.5
Less: accumulated depreciation	1,919.0	1,772.7
Partially-owned subsidiaries
Restricted cash	36.6	62.9
Property, plant and equipment, at cost	2,032.0	1,985.9
Less: accumulated depreciation	$ 472.0	$ 418.0