Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2016	[1]	Dec. 31, 2015
Unamortized discount	$ 35.8		$ 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	$ 183.3		$ 195.8
Property, plant and equipment, at cost	7,510.4		7,145.4
Less accumulated depreciation	1,903.8		1,797.4
Partially-owned subsidiaries
Restricted cash	80.3		89.9
Property, plant and equipment, at cost	1,981.0		1,980.1
Less accumulated depreciation	$ 340.1		$ 313.7

[1] (unaudited)