Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2015	[1]	Dec. 31, 2014
Unamortized discount	$ 52.3		$ 60.0
Preferred stock, shares authorized	1,500,000.0		1,500,000.0
Preferred stock, shares unissued	1,500,000.0		1,500,000.0
Common stock, shares authorized	400,000,000		200,000,000
Restricted cash	$ 197.3		$ 234.7
Property, plant, and equipment, at cost	6,955.4		6,586.0
Less accumulated depreciation	1,761.5		1,683.1
Partially-owned subsidiaries
Restricted cash	92.4		91.9
Property, plant, and equipment, at cost	2,258.9		2,261.2
Less accumulated depreciation	$ 292.9		$ 261.3

[1] (unaudited)