Disclosure - Goodwill and Other Intangible Assets - Summary of Goodwill Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill Beginning Balance, Gross	$ 21.3	$ 18.1	$ 18.1
Goodwill addition		3.2
No activity in period	0.0		0.0
Goodwill Ending Balance, Gross	21.3	21.3	18.1
Accumulated Impairments	(11.8)	(11.8)	(11.8)	$ (11.8)
Goodwill Beginning Balance, Net	9.5	6.3	6.3
Goodwill Ending Balance, Net	$ 9.5	$ 9.5	$ 6.3