Income Taxes - Details of Provision for Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
U.S. Federal expense (benefit)		$ (0.1)	$ (0.3)
State and local tax	$ 0.1	0.1	0.1
Total current	0.1		(0.2)
Deferred:
U.S. Federal tax	0.1	9.7	(1.6)
State and local tax	(0.1)	1.4	0.4
Total deferred		11.1	(1.2)
Total income tax expense (benefit)	$ 0.1	$ 11.1	$ (1.4)