Disclosure - Goodwill and Other Intangible Assets - Summary of Intangible Assets Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Finite Lived Intangible Assets [Line Items]
Cost amortizable intangible assets	$ 8.1	$ 8.1
Accumulated amortization intangible assets	2.9	1.7
Customer relationships [Member]
Finite Lived Intangible Assets [Line Items]
Cost amortizable intangible assets	4.9	4.9
Accumulated amortization intangible assets	1.5	1.1
Trademarks [Member]
Finite Lived Intangible Assets [Line Items]
Cost amortizable intangible assets	1.6	1.6
Accumulated amortization intangible assets	0.5	0.2
Other [Member]
Finite Lived Intangible Assets [Line Items]
Cost amortizable intangible assets	1.6	1.6
Accumulated amortization intangible assets	$ 0.9	$ 0.4