Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Accounts receivable, allowance for doubtful accounts (in dollars)	$ 1.6	$ 2.8
Property and equipment, accumulated depreciation (in dollars)	1,387.8	1,179.4
Intangible assets, accumulated amortization (in dollars)	$ 39.0	$ 29.6
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	77,000,000	76,600,000
Common stock, shares outstanding	61,400,000	67,400,000
Treasury stock, shares	15,600,000	9,200,000
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