Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Supplemental Cash Flow [Line Items]
Total interest paid	$ 11.1	$ 10.8
Income taxes paid	10.1	8.9
Income taxes recovered	(0.5)	(0.1)
Income taxes paid, net	9.6	8.8
Senior unsecured fixed rate notes
Supplemental Cash Flow [Line Items]
Total interest paid	4.7	4.7
Junior subordinated debentures
Supplemental Cash Flow [Line Items]
Total interest paid	5.4	5.0
Other indebtedness
Supplemental Cash Flow [Line Items]
Total interest paid	$ 1.0	$ 1.1