Goodwill and Other Intangible Assets - Summary of Goodwill Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill Beginning Balance, Gross	$ 21.3	$ 21.3	$ 21.3
No activity in period		0.0	0.0
Impairment	(9.5)
Goodwill Ending Balance, Gross	21.3	21.3	21.3
Accumulated Impairments	(21.3)	(11.8)	(11.8)	$ (11.8)
Goodwill Beginning Balance, Net	$ 9.5	9.5	9.5
Goodwill Ending Balance, Net		$ 9.5	$ 9.5