Shareholders' Equity - Reconciliation of Weighted Average Number of Shares (Detail) - shares shares in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Weighted Average Number of Shares Outstanding [Line Items]
Weighted average number of common shares outstanding	61.0	61.5	61.1
Director Plan and deferred stock units	0.2	0.2	0.2
ESOP	0.1	0.2	0.1
Common shares outstanding—basic	61.3	61.9	61.4
Common and potential common shares outstanding—diluted	62.7	62.6	62.3
Restricted Stock
Weighted Average Number of Shares Outstanding [Line Items]
Dilutive effect of stock awards	0.7	0.5	0.6
Stock Options
Weighted Average Number of Shares Outstanding [Line Items]
Dilutive effect of stock awards	0.7	0.2	0.3