Business and Significant Accounting Policies - Schedule of Interest Paid (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	$ 33.1	$ 31.3	$ 27.1
Income taxes paid	24.0	24.8	16.5
Income taxes recovered	(0.1)	0.0	(2.5)
Income taxes paid, net	23.9	24.8	14.0
Revolving credit facility [Member]
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	0.0	0.0	0.3
Senior Unsecured Fixed Rate Notes [Member]
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	9.3	9.3	9.3
Junior Subordinated Debentures [Member]
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	16.2	15.5	12.6
Other Indebtedness [Member]
Disclosure - Business and Significant Accounting Policies - Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) [Line Items]
Total interest paid	$ 7.6	$ 6.5	$ 4.9