Earnings Per Share Computation (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 294.5us-gaap_ProfitLoss	$ 689.8us-gaap_ProfitLoss	$ 530.1us-gaap_ProfitLoss
Less: Income allocated to participating securities	4.8	12.5	10.0
Net income available to common shareholders	$ 289.7	$ 677.3	$ 520.1
Weighted average common shares outstanding	75.6	79.3	83.3
Dilutive common share equivalents from share-based compensation plans	1.5	1.4	1.1
Weighted average common and common equivalent shares outstanding, assuming dilution	77.1	80.7	84.4

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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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The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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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 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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