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Components of Basic and Diluted Earnings Per Share (Detail) (USD $)
In Millions, except Per Share data, 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, 2012
Dec. 31, 2011
Dec. 31, 2010
Schedule of Earnings Per Share, Basic and Diluted, by Common Class                      
Net income available for common stockholders $ 112 $ 104 $ 112 $ 91 $ 98 $ 87 $ 103 $ 65 $ 419 $ 353 $ 301
Weighted average outstanding shares of common stock                 168.2 168.1 167.4
Dilutive effect of employee stock options and restricted stock                 3.5 3.8 3.0
Common stock and common stock equivalents                 171.7 171.9 170.4
Earnings per share: Basic $ 0.67 $ 0.62 $ 0.66 $ 0.54 $ 0.59 $ 0.52 $ 0.61 $ 0.39 $ 2.49 $ 2.10 $ 1.80
Earnings per share: Diluted $ 0.66 $ 0.60 $ 0.65 $ 0.53 $ 0.57 $ 0.51 $ 0.60 $ 0.38 $ 2.44 $ 2.05 $ 1.77