Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Numerator
Net Income	$ 229.9	$ 210.6
Denominator (000s)
Weighted Average Common Shares - Basic	229,429,600	239,619,400
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	949,200	311,000
Weighted Average Common Shares - Assuming Dilution	230,378,800	239,930,400
Net Income Per Common Share
Basic	$ 1.00	$ 0.88
Assuming Dilution	$ 1.00	$ 0.88

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

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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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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