Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018		Dec. 31, 2017
Unamortized discount	$ 0.3		$ 8.5
Preferred stock, shares authorized	1,500,000	[1]	1,500,000
Preferred stock, shares issued	0	[1]	0
Common stock, shares authorized	400,000,000.0	[1]	400,000,000.0
Restricted cash	$ 142.2	[1]	$ 195.2
Property, plant and equipment, at cost	8,864.1	[1]	8,385.2
Less accumulated depreciation	2,375.9	[1]	2,250.5
Partially-owned subsidiaries
Restricted cash	42.0	[1]	62.9
Property, plant and equipment, at cost	2,005.7	[1]	1,985.9
Less accumulated depreciation	$ 444.6	[1]	$ 418.0

[1] (unaudited)