Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Taxes [Line Items]
U.S. federal corporate statutory tax rate	21.00%		35.00%
Liability for uncertain tax positions totaled	$ 900		$ 900
Deferred income tax expense	(1,079)	$ (164)
Foreign Subsidiaries [Member]
Income Taxes [Line Items]
Deferred income tax expense	$ 500		$ 0
Federal Authorities [Member]
Income Taxes [Line Items]
Open tax years	2014 2015 2016 2017 2018
State and Foreign Authorities [Member]
Income Taxes [Line Items]
Open tax years	2012 2013 2014 2015 2016 2017 2018