Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Examination [Line Items]
Corporate income tax rate			21.00%		35.00%
Provisional income tax expense					$ 3,300
Income tax (provision) benefit	$ (906)	$ (3,505)	$ 4,669	$ (863)
Gross liability for tax positions	4,000		4,000
Change in unrecognized tax benefits reasonably possible	3,000		$ 3,000
Income tax examination years description			The Company is no longer subject to U.S. Federal income tax examinations for the years prior to 2015.
Interest and Penalties
Income Tax Examination [Line Items]
Gross liability for tax positions	$ 1,200		$ 1,200
State Jurisdiction | Minimum
Income Tax Examination [Line Items]
Income tax examination year			2013
State Jurisdiction | Maximum
Income Tax Examination [Line Items]
Income tax examination year			2017
Domestic
Income Tax Examination [Line Items]
Effective income tax rate	13.60%	42.90%	27.80%	42.90%
Foreign
Income Tax Examination [Line Items]
Effective income tax rate, change in enacted rate		78.80%		78.80%