Shareholders' Equity Reconciliation of Weighted Average Number of Shares (Detail) - shares shares in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average number of common shares outstanding (in shares)	61.6	61.4
Director Plan and deferred stock units (in shares)	0.3	0.2
ESOP (in shares)	0.1	0.3
Common shares outstanding - basic (in shares)	62.0	61.9
Common and potential common shares outstanding - diluted (in shares)	63.4	61.9
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dilutive effect of stock (in shares)	0.6	0.0
Share-based Payment Arrangement, Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dilutive effect of stock (in shares)	0.8	0.0

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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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.

+ References

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Number of employee stock ownership plan (ESOP) shares that have not been committed to be released determined by relating the portion of time within a reporting period that these shares have been outstanding to the total time in that period.

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This element represents the weighted average total number of shares issued throughout the period including the first (beginning balance outstanding) and last (ending balance outstanding) day of the period before considering any reductions (for instance, shares held in treasury) to arrive at the weighted average number of shares outstanding. Weighted average relates to the portion of time within a reporting period that common shares have been issued and outstanding to the total time in that period. Such concept is used in determining the weighted average number of shares outstanding for purposes of calculating earnings per share (basic).

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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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Number of shares of restricted stock determined by relating the portion of time within a reporting period that restricted shares have been outstanding to the total time in that period. Restricted shares are subject to sales, contractual, regulatory or other restrictions that prevent or inhibit the holder from freely disposing of them before the restriction ends.

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