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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