v2.4.0.6
Stockholders' equity (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Stock repurchase program                              
Stock repurchases (in shares) 14.2 9.7 17.4 21.0 86.0 45.4 12.9 0 20.5 6.6 10.3 29.1 62.3 144.3 66.5
Stock Repurchase Program [Member]
                              
Stock repurchase program                              
Stock repurchases 1,233 797 1,203 1,429 5,154 [1] 2,421 732 0 1,136 364 616 1,684 4,662 8,307 3,800
[1] Includes the repurchase of 83.3 million shares of our common stock at an average price paid per share of $60.08, including related expenses, for an aggregate cost of $5.0 billion, under a modified Dutch auction tender offer.