Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data	Jun. 30, 2011	Dec. 31, 2010
Current assets
Allowance for doubtful accounts - trade receivables	$ 9	$ 12
Accumulated depreciation	$ 1,261	$ 1,131
Shareholders' equity
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares, outstanding	0	0
Treasury stock, shares	22,543,031	22,269,278
Series A common stock, $0.0001 par value, 400,000,000 shares authorized (2011: 178,886,161 issued and 156,343,130 outstanding; 2010: 178,028,571 issued and 155,759,293 outstanding)
Shareholders' equity
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares issued	178,886,161	178,028,571
Common stock, shares outstanding	156,343,130	155,759,293
Series B common stock, $0.0001 par value, 100,000,000 shares authorized (2011 and 2010: 0 issued and outstanding)
Shareholders' equity
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	0	0
Common stock, shares outstanding	0	0