Income Taxes - Reconciliation of Gross Amount of Unrecognized Tax Benefits Excluding Accrued Interest and Penalties (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Balance at beginning of period	$ 409	$ 409	$ 434
Increases related to prior year tax positions	0	0	0
Decrease related to prior year tax positions	0	0	0
Increases related to current year tax positions	0	0	0
Settlements	0	0	0
Lapse of statute of limitations	(409)	0	(25)
Balance at end of period	$ 0	$ 409	$ 409