Business and Significant Accounting Policies - Schedule of Interest Paid (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	$ 21.8	$ 26.4	$ 33.1
Income taxes paid	43.0	47.7	24.0
Income taxes recovered	(2.6)	(1.8)	(0.1)
Income taxes paid, net	40.4	45.9	23.9
Senior unsecured fixed rate notes
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	9.3	9.3	9.3
Junior subordinated debentures
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	10.0	11.9	16.2
Other indebtedness
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	$ 2.5	$ 5.2	$ 7.6