Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Accounts receivable, allowance for doubtful accounts (in dollars)	$ 1.6	$ 1.6
Property and equipment, accumulated depreciation (in dollars)	1,462.9	1,387.8
Intangible assets, accumulated amortization (in dollars)	$ 43.3	$ 39.0
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	77,600,000	77,000,000
Common stock, shares outstanding	62,000,000	61,400,000
Treasury stock, shares	15,600,000	15,600,000
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
Preferred stock, shares outstanding	80,000	80,000