Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2016	[1]	Dec. 31, 2015
Unamortized discount	$ 31.5		$ 44.2
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
Restricted cash	$ 183.9		$ 195.8
Property, plant and equipment, at cost	7,713.9		7,145.4
Less accumulated depreciation	1,950.7		1,797.4
Partially-owned subsidiaries
Restricted cash	81.7		89.9
Property, plant and equipment, at cost	1,978.6		1,980.1
Less accumulated depreciation	$ 351.9		$ 313.7

[1] (unaudited)