Income Taxes - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended			12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2018
Income Tax Disclosure [Line Items]
Income tax provisions		$ 1,035,000		$ 1,727,000
Discrete tax expense		$ 1,200,000
U.S. federal statutory tax rate	21.00%	24.50%		35.00%
State income tax reserve reversals related to statutory filing		$ 100,000		$ 200,000
Unrecognized tax benefits excluding interest and penalties that would impact effective tax rate	$ 1,400,000	1,400,000	$ 1,200,000
Undistributed earnings in foreign subsidiaries	$ 0	0	$ 0
IRS [Member]
Income Tax Disclosure [Line Items]
Income tax examination, year under examination			2016
Expected Statutory Income Tax Rate [Member]
Income Tax Disclosure [Line Items]
U.S. federal statutory tax rate					24.50%
Expiring Stock Option Awards [Member]
Income Tax Disclosure [Line Items]
Discrete tax (benefit) expense		$ (200,000)		$ 300,000