Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Unamortized discount	$ 56.2	[1]	$ 60.0
Preferred stock, shares authorized	1,500,000	[1]	1,500,000
Preferred stock, shares unissued	1,500,000	[1]	1,500,000
Common stock, shares authorized	200,000,000	[1]	200,000,000
Restricted cash	211.1	[1]	234.7
Property, plant, and equipment, at cost	6,870.3	[1]	6,586.0
Less accumulated depreciation	1,721.4	[1]	1,683.1
Partially-owned subsidiaries
Restricted cash	89.1	[1]	91.9
Property, plant, and equipment, at cost	2,260.6	[1]	2,261.2
Less accumulated depreciation	$ 277.2	[1]	$ 261.3

[1] (unaudited)