v2.4.0.6
Stockholders' equity (Details Textual) (USD $)
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 13, 2012
Dec. 07, 2012
Oct. 10, 2012
Sep. 07, 2012
Jul. 19, 2012
Jun. 07, 2012
Mar. 15, 2012
Mar. 07, 2012
Dec. 15, 2011
Dec. 08, 2011
Oct. 13, 2011
Sep. 08, 2011
Jul. 28, 2011
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 [Line Items]                                                        
Stock repurchases (in shares)                           14,200,000 9,700,000 17,400,000 21,000,000 86,000,000 45,400,000 12,900,000 0 20,500,000 6,600,000 10,300,000 29,100,000 62,300,000 144,300,000 66,500,000
Share repurchase authorized amount                                                   $ 2,000,000,000    
Amount available for stock repurchases under a board approved stock repurchase plan                                                   2,300,000,000    
Common stock, dividends declared per share (in usd per share) $ 0.47   $ 0.36   $ 0.36   $ 0.36   $ 0.36   $ 0.28   $ 0.28                              
Dividends paid per share (in usd per share)   $ 0.36   $ 0.36   $ 0.36   $ 0.36   $ 0.28   $ 0.28                                
Income taxes expense or (benefit) for unrealized gains and losses for cash flow hedges                                                   8,000,000 (20,000,000) 71,000,000
Income tax expense or (benefit) reclassification adjustments to income for cash flow hedges                                                   (49,000,000) 41,000,000 (16,000,000)
Income taxes expense or (benefit) for unrealized gains and losses for available-for-sale securities                                                   87,000,000 45,000,000 60,000,000
Income tax expense or (benefit) for reclassification adjustments to income for available-for-sale securities                                                   (49,000,000) (59,000,000) (35,000,000)
Preferred stock shares authorized (in shares)                           5,000,000                       5,000,000    
Preferred stock, par value (in usd per share)                           $ 0.0001                       $ 0.0001    
Preferred stock, shares issued (in shares)                           0       0               0 0  
Preferred stock, shares outstanding (in shares)                           0       0               0 0  
Modified Dutch Auction Tender Offer [Member]
                                                        
Stock Repurchase Program [Line Items]                                                        
Stock repurchases (in shares)                                                     83,300,000  
Stock repurchased price per share (in usd per share)                                                     $ 60.08  
Stock repurchases                                                     $ 5,000,000,000