Supplemental Cash Flow Information - Schedule of Interest Paid and Income Taxes Paid (Recovered) (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Supplemental Cash Flow [Line Items]
Total interest paid	$ 16.8	$ 15.4
Income taxes paid	1.3	8.2
Income taxes recovered	0.0	0.0
Income taxes paid, net	1.3	8.2
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	4.7	4.7
Junior Subordinated Debentures [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	8.3	7.1
Other Indebtedness [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	$ 3.8	$ 3.3