Supplemental Cash Flow Information - Schedule of Interest Paid (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Supplemental Cash Flow [Line Items]
Total interest paid	$ 12.6	$ 9.7
Revolving Credit Facility [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	0.3	0.0
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	6.0	3.8
Other Indebtedness [Member]
Supplemental Cash Flow [Line Items]
Total interest paid	$ 1.6	$ 1.2