Condensed Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Accounts receivable, allowance for doubtful accounts (in dollars)	$ 1.5	$ 0.5
Property and equipment, accumulated depreciation (in dollars)	985.8	1,095.5
Intangible assets, accumulated amortization (in dollars)	$ 22.1	$ 19.5
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	76,500,000	76,100,000
Common stock, shares outstanding	72,500,000	73,200,000
Treasury stock, shares	4,000,000	2,900,000
Series A Convertible Preferred Stock
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	80,000	80,000
Preferred stock, shares issued	80,000	80,000