Allowance for Doubtful Accounts - Summary of Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Balance at beginning of year	$ 0.7	$ 0.8	$ 0.6
Provision charged to expense	0.2		0.3
Write-offs, less recoveries		(0.1)	(0.1)
Balance at end of year	$ 0.9	$ 0.7	$ 0.8