Income Tax Asset (Liability) (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Net income tax asset (liability)	$ (1,934)	$ (1,934)
State and Local Jurisdiction
Income Taxes [Line Items]
Current federal income tax asset (liability)	(108)	(108)
Deferred federal income tax asset (liability)	(67)	(67)
Internal Revenue Service (IRS)
Income Taxes [Line Items]
Current federal income tax asset (liability)	1,483
Deferred federal income tax asset (liability)	$ (3,242)	$ (1,759)