Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	[1]	Dec. 31, 2016
Unamortized discount	$ 13.2		$ 27.1
Preferred stock, shares authorized	1,500,000		1,500,000
Preferred stock, shares issued	0		0
Common stock, shares authorized	400,000,000		400,000,000.0
Restricted cash	$ 176.3		$ 178.2
Property, plant and equipment, at cost	8,274.9		7,981.0
Less accumulated depreciation	2,204.8		2,014.2
Partially-owned subsidiaries
Restricted cash	66.6		78.4
Property, plant and equipment, at cost	1,983.9		1,979.8
Less accumulated depreciation	$ 404.7		$ 364.9

[1] (unaudited)