Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Supplemental Cash Flow [Line Items]
Total interest paid	$ 7.9	$ 8.8
Income taxes paid	0.3	0.2
Income taxes recovered	0.0	0.0
Income taxes paid, net	0.3	0.2
Senior Unsecured Fixed Rate Notes [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	2.3	2.3
Junior Subordinated Debentures [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	3.7	4.1
Other Indebtedness [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	$ 1.9	$ 2.4