Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Supplemental Cash Flow [Line Items]
Total interest paid	$ 26.5	$ 23.1
Income taxes paid	4.8	18.1
Income taxes recovered	(0.1)	0.0
Income taxes paid, net	4.7	18.1
Senior Unsecured Fixed Rate Notes [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	7.0	7.0
Junior Subordinated Debentures [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	12.3	11.2
Other Indebtedness [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	$ 7.2	$ 4.9