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Earnings Per Share (Summary of Computation of Basic and Diluted Earnings Per Share) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Earnings Per Share [Abstract]        
Net income attributable to common shareholders $ 113 $ 89 $ 244 $ 266
Weighted-average common shares outstanding for basic earnings per share 167,337,700 166,235,266 166,476,927 169,516,767
Employee equity awards 3,773,791 4,284,300 3,854,863 3,970,984
Contingent issuance of common stock, shares 992,247 [1]   330,749 [1]  
3.75% convertible notes       20,891 [2]
Weighted-average common shares outstanding for diluted earnings per share 172,103,738 170,519,566 170,662,539 173,508,642
Basic earnings per share $ 0.68 $ 0.53 $ 1.46 $ 1.57
Diluted earnings per share $ 0.66 $ 0.52 $ 1.43 $ 1.53
[1] See “Acquisition of eSpeed for Trading of U.S. Treasuries,” of Note 4, “Acquisitions and Divestiture,” for further discussion.
[2] In June 2012, the remaining $0.5 million of our 3.75% convertible notes outstanding was converted into 34,482 shares of common stock in accordance with the terms of the notes.