Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Supplemental Cash Flow [Line Items]
Total interest paid	$ 9.0	$ 8.6
Income taxes paid	0.0	0.4
Income taxes recovered	0.0	(0.1)
Income taxes paid, net	0.0	0.3
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	6.1	5.5
Other indebtedness
Supplemental Cash Flow [Line Items]
Total interest paid	$ 0.6	$ 0.8