Goodwill and Identifiable Intangible Assets, Net - Identifiable intangible assets (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill And Other Intangible Assets
Impairment	$ (10.1)	$ (10.1)	$ (0.1)	$ 0.0
Accumulated amortization	(6.9)	(6.9)	(4.5)
Identifiable intangible assets, net			12.5
Customer Relationships
Goodwill And Other Intangible Assets
Identifiable intangible assets gross	$ 17.0	$ 17.0	$ 17.0