Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Supplemental Cash Flow [Line Items]
Total interest paid	$ 21.0	$ 26.5
Income taxes paid	46.0	4.8
Income taxes recovered	(1.6)	(0.1)
Income taxes paid, net	44.4	4.7
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	9.4	12.3
Other indebtedness
Supplemental Cash Flow [Line Items]
Total interest paid	$ 4.6	$ 7.2