Segment Information - Reconciliation of U.S. GAAP Income Before Income Taxes to Adjusted EBITDA (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting [Abstract]
Adjusted EBITDA		$ 5,165	$ 5,303	$ 29,149	$ 34,853
Stock-based compensation expense		(2,080)	(434)	(3,292)	(650)
Interest expense		(590)	(45)	(860)	(61)
Interest income and other	[1]	(508)	(536)	(709)	719
Depreciation and amortization		(5,274)	(3,982)	(10,468)	(7,525)
(Loss) income before income taxes		$ (3,287)	$ 306	$ 13,820	$ 27,336	$ 27,024	$ (36,411)

[1] ^{Includes an impairment charge for royalty contracts resulting in $0.8 million and $1.0 million of expense for the three and six months ended June 30, 2019, respectively, and $0.9 million and $1.8 million of expense for the three and six months ended June 30, 2018, respectively. Includes a non-recurring adjustment for a change in estimated legal expenses resulting in $2.0 million of income for the six months ended June 30, 2018}