Computations of Net Income Per Common Share (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
BASIC NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding	235,829,790	239,265,313	240,417,805
Net Income	$ 811.8us-gaap_NetIncomeLoss	$ 731.3us-gaap_NetIncomeLoss	$ 687.3us-gaap_NetIncomeLoss
Less: Dividends on Preferred Stock	9.5
Net Income Applicable to Common Stock	802.3	731.3	687.3
Less: Earnings Allocated to Participating Securities	13.3	11.9	10.0
Earnings Allocated to Common Shares Outstanding	789.0	719.4	677.3
Basic Net Income Per Common Share	$ 3.34us-gaap_EarningsPerShareBasic	$ 3.01us-gaap_EarningsPerShareBasic	$ 2.82us-gaap_EarningsPerShareBasic
DILUTED NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding	235,829,790	239,265,313	240,417,805
Plus Dilutive Effect of Share-based Compensation	1,890,465	1,289,527	463,439
Average Common and Potential Common Shares	237,720,255	240,554,840	240,881,244
Earnings Allocated to Common and Potential Common Shares	$ 789.0	$ 719.5	$ 677.3
Diluted Net Income Per Common Share	$ 3.32us-gaap_EarningsPerShareDiluted	$ 2.99us-gaap_EarningsPerShareDiluted	$ 2.81us-gaap_EarningsPerShareDiluted

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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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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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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The amount of preferred stock dividends that is an adjustment to net income apportioned to common stockholders.

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Amount of undistributed earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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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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