Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
		Mar. 31, 2025	Mar. 31, 2024
Earnings Per Share [Abstract]
Income (Loss) from Continuing Operations, Per Basic Share		$ 2.22	$ 2.07
Calculation of Basic Earnings Per Share:
Net Income		$ 50.9	$ 46.2
Weighted shares outstanding (shares)	[1]	22,957,000	22,250,000
Basic Earnings Per Share (in dollars per share)		$ 2.22	$ 2.07
Reconciliation of Numerator:
Net Income		$ 50.9	$ 46.2
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	[1]	22,957,000	22,250,000
Effect of dilutive securities:
Share-based compensation (shares)		84,000	56,000
Adjusted denominator-Diluted (shares)		23,041,000	22,306,000
Income (Loss) from Continuing Operations, Per Diluted Share		$ 2.21	$ 2.07
Diluted Earnings Per Share (in dollars per share)		$ 2.21	$ 2.07
Net Income		$ 50.9	$ 46.2
Basic (shares)	[1]	22,957,000	22,250,000
Income (Loss) from Continuing Operations, Per Basic Share		$ 2.22	$ 2.07
Basic (in dollars per share)		$ 2.22	$ 2.07
Share-based compensation (shares)		84,000	56,000
Diluted (shares)		23,041,000	22,306,000
Income (Loss) from Continuing Operations, Per Diluted Share		$ 2.21	$ 2.07
Diluted (in dollars per share)		$ 2.21	$ 2.07
Retained Earnings [Member]
Calculation of Basic Earnings Per Share:
Net Income		$ 50.9	$ 46.2
Reconciliation of Numerator:
Net Income		50.9	46.2
Net Income		$ 50.9	$ 46.2

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