Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	[1]	Dec. 31, 2017
Unamortized discount			$ 8.5
Preferred stock, shares authorized	1,500,000		1,500,000
Preferred stock, shares issued	0		0
Common stock, shares authorized	400,000,000.0		400,000,000.0
Restricted cash	$ 138.5		$ 195.2
Property, plant and equipment, at cost	8,978.1		8,385.2
Less accumulated depreciation	2,440.1		2,250.5
Partially-owned subsidiaries
Restricted cash	38.6		62.9
Property, plant and equipment, at cost	2,019.0		1,985.9
Less accumulated depreciation	$ 458.2		$ 418.0

[1] (unaudited)