Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current Assets
Allowance for doubtful accounts - trade receivables	$ 11	$ 9
Accumulated depreciation	$ 1,665	$ 1,506
Stockholders' 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	26,089,177	23,986,836
Series A common stock, $0.0001 par value, 400,000,000 shares authorized (2013: 183,781,497 issued and 157,692,320 outstanding; 2012: 183,629,237 issued and 159,642,401 outstanding) [Member]
Stockholders' Equity
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares issued	183,781,497	183,629,237
Common stock, shares outstanding	157,692,320	159,642,401
Series B common stock, $0.0001 par value, 100,000,000 shares authorized (2013 and 2012: 0 issued and outstanding) [Member]
Stockholders' 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