Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Millions, $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Preferred stock, shares authorized	1.5	1.5
Preferred Stock, Shares Unissued	1.5	1.5
Common stock, shares authorized	400.0	400.0
Restricted cash	$ 114.2	$ 171.6
Property, plant and equipment, at cost	8,723.2	8,253.4
Less accumulated depreciation	1,976.4	1,919.0
Partially-owned subsidiaries
Restricted cash	29.0	36.6
Property, plant and equipment, at cost	2,042.3	2,032.0
Less accumulated depreciation	$ 485.9	$ 472.0