Income Taxes - Details of Provision for Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
U.S. Federal expense (benefit)	$ (0.3)	$ (0.1)	$ 0.1
State and local tax	0.1	0.4	0.3
Total current	(0.2)	0.3	0.4
Deferred:
U.S. Federal tax	(1.6)	3.7	6.9
State and local tax	0.4	(0.8)	(0.1)
Total deferred	(1.2)	2.9	6.8
Total income tax expense (benefit)	$ (1.4)	$ 3.2	$ 7.2