Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2016	[1]	Dec. 31, 2015
Unamortized discount	$ 40.0		$ 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	$ 185.6		$ 195.8
Property, plant, and equipment	7,372.2		7,145.4
Less accumulated depreciation	1,848.7		1,797.4
Partially-owned subsidiaries
Restricted cash	83.6		89.9
Property, plant, and equipment	1,981.6		1,980.1
Less accumulated depreciation	$ 327.6		$ 313.7

[1] (unaudited)