Income Taxes - Details of Provision for Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
U.S. Federal expense (benefit)	$ (0.1)	$ (0.3)	$ (0.1)
State and local tax	0.1	0.1	0.4
Total current		(0.2)	0.3
Deferred:
U.S. Federal tax	9.7	(1.6)	3.7
State and local tax	1.4	0.4	(0.8)
Total deferred	11.1	(1.2)	2.9
Total income tax expense (benefit)	$ 11.1	$ (1.4)	$ 3.2