Accounting Policies - Computation of Numerators and Denominators Used in Basic and Diluted Income (loss) per Share Amounts (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net (loss) income	$ (3,120)	$ 13,019	$ (1,080)	$ 24,684	$ 9,899	$ 23,604	$ 15,535	$ (101,941)
Denominator:
Denominator for basic (loss) income per share— weighted average shares	71,373		65,580		71,081	64,614
Effect of dilutive securities, stock options and RSUs					5,936	8,267
Denominator for dilutive (loss) income per share	71,373		65,580		77,017	72,881
Net (loss) income per share attributable to common stockholders, basic	$ (0.04)		$ (0.02)		$ 0.14	$ 0.37	$ 0.23	$ (1.94)
Net (loss) income per share attributable to common stockholders, diluted	$ (0.04)		$ (0.02)		$ 0.13	$ 0.32	$ 0.21	$ (1.94)