Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Supplemental Cash Flow [Line Items]
Total interest paid	$ 14.8	$ 16.8
Income taxes paid	1.6	1.3
Income taxes recovered	(0.7)	0.0
Income taxes paid, net	0.9	1.3
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	6.7	8.3
Other Indebtedness
Supplemental Cash Flow [Line Items]
Total interest paid	$ 3.4	$ 3.8