Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Earnings Per Share [Abstract]
Income (Loss) from Continuing Operations, Per Basic Share		$ 1.03	$ 0.82	$ 3.23	$ 2.89
Calculation of Basic Earnings Per Share:
Net Income		$ 23.9	$ 18.2	$ 74.8	$ 64.4
Weighted shares outstanding (shares)	[1]	23,307,000	22,284,000	23,133,000	22,267,000
Basic Earnings Per Share (in dollars per share)		$ 1.03	$ 0.82	$ 3.23	$ 2.89
Reconciliation of Numerator:
Net Income		$ 23.9	$ 18.2	$ 74.8	$ 64.4
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	[1]	23,307,000	22,284,000	23,133,000	22,267,000
Effect of dilutive securities:
Share-based compensation (shares)		95,000	51,000	90,000	53,000
Adjusted denominator-Diluted (shares)		23,402,000	22,335,000	23,223,000	22,320,000
Income (Loss) from Continuing Operations, Per Diluted Share		$ 1.02	$ 0.82	$ 3.22	$ 2.89
Diluted Earnings Per Share (in dollars per share)		$ 1.02	$ 0.82	$ 3.22	$ 2.89
Net Income		$ 23.9	$ 18.2	$ 74.8	$ 64.4
Basic (shares)	[1]	23,307,000	22,284,000	23,133,000	22,267,000
Income (Loss) from Continuing Operations, Per Basic Share		$ 1.03	$ 0.82	$ 3.23	$ 2.89
Basic (in dollars per share)		$ 1.03	$ 0.82	$ 3.23	$ 2.89
Share-based compensation (shares)		95,000	51,000	90,000	53,000
Diluted (shares)		23,402,000	22,335,000	23,223,000	22,320,000
Income (Loss) from Continuing Operations, Per Diluted Share		$ 1.02	$ 0.82	$ 3.22	$ 2.89
Diluted (in dollars per share)		$ 1.02	$ 0.82	$ 3.22	$ 2.89
Retained Earnings [Member]
Calculation of Basic Earnings Per Share:
Net Income		$ 23.9		$ 74.8	$ 64.4
Reconciliation of Numerator:
Net Income		23.9		74.8	64.4
Net Income		$ 23.9		$ 74.8	$ 64.4

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