Goodwill and Other Intangible Assets - Summary of Goodwill Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill Beginning Balance, Gross	$ 21.3	$ 21.3	$ 21.3
No activity in period	0.0	0.0	0.0
Goodwill Ending Balance, Gross	21.3	21.3	21.3
Accumulated Impairments	(11.8)	(11.8)	(11.8)	$ (11.8)
Goodwill Beginning Balance, Net	9.5	9.5	9.5
Goodwill Ending Balance, Net	$ 9.5	$ 9.5	$ 9.5