Income Tax Asset (Liability) (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Net income tax asset (liability)	$ (2,461)	$ (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)	427
Deferred federal income tax asset (liability)	$ (2,713)	$ (1,759)