Computations of Net Income Per Common Share (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	239,167,559	241,090,093
Net Income Applicable to Common Stock	$ 164.0	$ 161.2
Less: Earnings Allocated to Participating Securities	2.5	2.1
Earnings Allocated to Common Shares Outstanding	161.5	159.1
Basic Net Income Per Common Share	$ 0.68	$ 0.66
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	239,167,559	241,090,093
Plus: Dilutive Effect of Share-based Compensation	1,021,656	466,003
Average Common and Potential Common Shares	240,189,215	241,556,096
Earnings Allocated to Common and Potential Common Shares	$ 161.5	$ 160.0
Diluted Net Income Per Common Share	$ 0.67	$ 0.66

Net Income (Loss) Allocated to Common Stockholders, Basic

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Net Income or Loss Available to Common Stockholders plus adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

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Earnings allocated (not distributed) to participating securities under the two-class method to the extent that participating securities may share in earnings as if all of the earnings for the period had been distributed.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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