Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
Supplemental Cash Flow [Line Items]
Total interest paid	$ 17.0	$ 16.0
Income taxes paid	26.2	30.7
Income taxes recovered	(0.5)	(1.2)
Income taxes paid, net	25.7	29.5
Senior unsecured fixed rate notes
Supplemental Cash Flow [Line Items]
Total interest paid	7.0	7.0
Junior subordinated debentures
Supplemental Cash Flow [Line Items]
Total interest paid	9.0	7.5
Other indebtedness
Supplemental Cash Flow [Line Items]
Total interest paid	$ 1.0	$ 1.5