Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Supplemental Cash Flow [Line Items]
Total interest paid	$ 8.6	$ 5.6
Income taxes paid	0.4	0.3
Income taxes recovered	(0.1)	(0.2)
Income taxes paid, net	0.3	0.1
Senior unsecured fixed rate notes
Supplemental Cash Flow [Line Items]
Total interest paid	2.3	2.3
Junior subordinated debentures
Supplemental Cash Flow [Line Items]
Total interest paid	5.5	2.5
Other indebtedness
Supplemental Cash Flow [Line Items]
Total interest paid	$ 0.8	$ 0.8