Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Earnings Per Share [Abstract]
Income (Loss) from Continuing Operations, Per Basic Share		$ 0.82	$ 0.78	$ 4.05
Calculation of Basic Earnings Per Share:
Net Income		$ 19.4	$ 17.5	$ 94.2	$ 81.9
Weighted shares outstanding (shares)	[1]	23,526,000	22,501,000	23,265,000	22,346,000
Basic Earnings Per Share (in dollars per share)		$ 0.82	$ 0.78	$ 4.05	$ 3.67
Reconciliation of Numerator:
Net Income		$ 19.4	$ 17.5	$ 94.2	$ 81.9
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	[1]	23,526,000	22,501,000	23,265,000	22,346,000
Effect of dilutive securities:
Share-based compensation (shares)		103,000	63,000	95,000	56,000
Adjusted denominator-Diluted (shares)		23,629,000	22,564,000	23,360,000	22,402,000
Income (Loss) from Continuing Operations, Per Diluted Share		$ 0.82	$ 0.78	$ 4.03	$ 3.66
Diluted Earnings Per Share (in dollars per share)		$ 0.82	$ 0.78	$ 4.03	$ 3.66
Net Income		$ 19.4	$ 17.5	$ 94.2	$ 81.9
Basic (shares)	[1]	23,526,000	22,501,000	23,265,000	22,346,000
Income (Loss) from Continuing Operations, Per Basic Share		$ 0.82	$ 0.78	$ 4.05
Basic (in dollars per share)		$ 0.82	$ 0.78	$ 4.05	$ 3.67
Share-based compensation (shares)		103,000	63,000	95,000	56,000
Diluted (shares)		23,629,000	22,564,000	23,360,000	22,402,000
Income (Loss) from Continuing Operations, Per Diluted Share		$ 0.82	$ 0.78	$ 4.03	$ 3.66
Diluted (in dollars per share)		$ 0.82	$ 0.78	$ 4.03	$ 3.66
Retained Earnings [Member]
Calculation of Basic Earnings Per Share:
Net Income		$ 19.4		$ 94.2	$ 81.9
Reconciliation of Numerator:
Net Income		19.4		94.2	81.9
Net Income		$ 19.4		$ 94.2	$ 81.9

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