Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Supplemental Cash Flow [Line Items]
Total interest paid	$ 23.1	$ 19.7
Income taxes paid	18.1	10.1
Income taxes recovered	0.0	(2.4)
Income taxes paid, net	18.1	7.7
Revolving Credit Facility [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	0.0	0.3
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	11.2	9.4
Other Indebtedness [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	$ 4.9	$ 3.0