Option Activity Under Plan (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Plans [Line Items]
Weighted Average Remaining Contractual Term (In years), Outstanding	2 years 3 months 18 days	5 years 2 months 12 days
Employee Stock Option [Member]
Share Based Compensation Plans [Line Items]
Outstanding	707	575
Granted	68	132
Forfeited	(87)	0
Expired	(46)
Exercised		0
Outstanding	642	707	575
Vested and expected to vest	627	673
Exercisable	562	544
Weighted - Average Exercise Price, Outstanding	$ 11.6	$ 10.5
Weighted - Average Exercise Price, Granted	7.5	25.4
Weighted - Average Exercise Price, Forfeited	16.50	0
Weighted - Average Exercise Price,Expired	21.30
Weighted - Average Exercise Price, Exercised		0
Weighted - Average Exercise Price, Outstanding	13.3	11.6	$ 10.5
Weighted - Average Exercise Price, Vested and expected to vest	11.5	12.7
Weighted - Average Exercise Price, Exercisable	$ 10.7	$ 10.1
Weighted Average Remaining Contractual Term (In years), Outstanding	4 years	5 years 2 months 12 days	5 years 3 months 18 days
Weighted Average Remaining Contractual Term (In years), Granted	9 years 3 months 18 days	9 years 3 months 18 days
Weighted Average Remaining Contractual Term (In years), Vested and expected to vest	3 years 9 months 18 days	5 years
Weighted Average Remaining Contractual Term (In years), Exercisable	3 years 3 months 18 days	4 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding	$ 0	$ 1,972
Aggregate Intrinsic Value, Outstanding	2,241	0	$ 1,972
Aggregate Intrinsic Value, Vested and expected to vest	0	2,239
Aggregate Intrinsic Value, Exercisable	$ 0	$ 2,236