Income Taxes - Details of Provision for Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
U.S. Federal expense (benefit)	$ 0.1	$ (0.1)	$ (0.1)
State and local tax	0.3	0.1	$ 0.1
Total current	0.4
Deferred:
U.S. Federal tax	6.9	(16.4)
State and local tax	(0.1)	(0.8)
Total deferred	6.8	(17.2)
Total income tax	$ 7.2	$ (17.2)