Schedule II - Valuation and Qualifying Accounts (Detail) - Allowance for Doubtful Accounts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 0	$ 15	$ 28
Charged to Costs and Expenses		2	(6)
Deductions/ Write-offs (Recoveries)		17	7
Balance at End of Period	$ 0	$ 0	$ 15