Option Activity under Stock-Based Compensation Plans (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Shares Underlying Outstanding Options
Beginning balance | shares	3,555
Options granted | shares	2,110
Options exercised | shares	(30)
Ending balance | shares	5,481
Vested and expected to vest at June 30, 2018 | shares	5,215
Exercisable at June 30, 2018 | shares	2,659
Weighted-Average Exercise Price Per Share
Beginning balance | $ / shares	$ 19.56
Options granted | $ / shares	16.87
Options exercised | $ / shares	14.25
Ending balance | $ / shares	18.50
Vested and expected to vest at June 30, 2018 | $ / shares	18.57
Exercisable at June 30, 2018 | $ / shares	$ 20.04
Weighted-Average Remaining Contractual Term (years)
Balance at June 30, 2018	5 years 10 months 28 days
Vested and expected to vest at June 30, 2018	5 years 10 months 13 days
Exercisable at June 30, 2018	5 years 3 months 14 days
Aggregate Intrinsic Value
Balance at June 30, 2018 | $	$ 2,373
Vested and expected to vest at June 30, 2018 | $	2,302
Exercisable at June 30, 2018 | $	$ 1,420
Unvested
Shares Underlying Outstanding Options
Options cancelled | shares	(110)
Weighted-Average Exercise Price Per Share
Options cancelled | $ / shares	$ 13.30
Vested
Shares Underlying Outstanding Options
Options expired | shares	(44)
Weighted-Average Exercise Price Per Share
Options expired | $ / shares	$ 40.41