Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Supplemental Cash Flow [Line Items]
Total interest paid	$ 15.4	$ 12.6
Income taxes paid	8.2	9.9
Income taxes recovered	0.0	(2.2)
Income taxes paid, net	8.2	7.7
Revolving Credit Facility [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	0.3	0.3
Senior Unsecured Fixed Rate Notes [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	4.7	4.7
Junior Subordinated Debentures [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	7.1	6.0
Other Indebtedness [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	$ 3.3	$ 1.6