Shareholders' Equity Reconciliation of Weighted Average Number of Shares (Detail) - shares shares in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average number of common shares outstanding (in shares)	61.0	61.3	61.3	61.3
Director Plan and deferred stock units (in shares)	0.2	0.2	0.2	0.2
ESOP (in shares)	0.1	0.1	0.1	0.2
Common shares outstanding - basic (in shares)	61.3	61.6	61.6	61.7
Common and potential common shares outstanding - diluted (in shares)	62.8	61.6	63.1	61.7
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dilutive effect of stock (in shares)	0.7	0.0	0.7	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	0.8	0.0