UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Thousands	Common stock	Additional paid in capital	Accumulated foreign currency translation adjustments	Accumulated unrealized gains (losses) on available-for-sale debt securities	Accumulated deficit	Total
Balance at the beginning of the period at Dec. 31, 2018	$ 939	$ 574,208	$ (9,607)	$ (156)	$ (318,499)	$ 246,885
Balance at the beginning of the period, shares at Dec. 31, 2018	627,454,270
Increase (Decrease) in Stockholders' Equity
Net loss					(27,412)	(27,412)
Issuance of shares upon exercise of stock options	$ 1	35				36
Issuance of shares upon exercise of stock options (in shares)	840,432
Other comprehensive loss before reclassifications
Foreign currency translation adjustments			(3,543)			(3,543)
Unrealized holding gains on available-for-sale debt securities, net of tax				210		210
Share-based compensation expense		3,479				3,479
Balance at the end of the period at Mar. 31, 2019	$ 940	577,722	(13,150)	54	(345,911)	219,655
Balance at the end of the period, shares at Mar. 31, 2019	628,294,702
Balance at the beginning of the period at Dec. 31, 2018	$ 939	574,208	(9,607)	(156)	(318,499)	246,885
Balance at the beginning of the period, shares at Dec. 31, 2018	627,454,270
Increase (Decrease) in Stockholders' Equity
Net loss						(68,499)
Other comprehensive loss before reclassifications
Foreign currency translation adjustments						1,299
Unrealized holding gains on available-for-sale debt securities, net of tax						278
Reclassification from accumulated other comprehensive income of losses on available-for-sale debt securities included in net income, net of tax						(13)
Balance at the end of the period at Jun. 30, 2019	$ 943	581,245	(8,308)	109	(386,998)	186,991
Balance at the end of the period, shares at Jun. 30, 2019	630,672,578
Balance at the beginning of the period at Mar. 31, 2019	$ 940	577,722	(13,150)	54	(345,911)	219,655
Balance at the beginning of the period, shares at Mar. 31, 2019	628,294,702
Increase (Decrease) in Stockholders' Equity
Net loss					(41,087)	(41,087)
Issuance of shares upon exercise of stock options	$ 3	327				330
Issuance of shares upon exercise of stock options (in shares)	2,377,876
Other comprehensive loss before reclassifications
Foreign currency translation adjustments			4,842			4,842
Unrealized holding gains on available-for-sale debt securities, net of tax				68		68
Reclassification from accumulated other comprehensive income of losses on available-for-sale debt securities included in net income, net of tax				(13)		(13)
Share-based compensation expense		3,196				3,196
Balance at the end of the period at Jun. 30, 2019	$ 943	581,245	(8,308)	109	(386,998)	186,991
Balance at the end of the period, shares at Jun. 30, 2019	630,672,578
Balance at the beginning of the period at Dec. 31, 2019	$ 943	585,623	(7,302)	38	(455,664)	$ 123,638
Balance at the beginning of the period, shares at Dec. 31, 2019	631,003,568					631,003,568
Increase (Decrease) in Stockholders' Equity
Net loss					(28,167)	$ (28,167)
Issuance of shares upon exercise of stock options	$ 6	888				894
Issuance of shares upon exercise of stock options (in shares)	4,610,772
Issuance of shares upon completion of public offering, net of issuance costs	$ 190	90,360				90,550
Issuance of shares upon completion of public offering, net of issuance costs (in shares)	144,900,000
Other comprehensive loss before reclassifications
Foreign currency translation adjustments			17,911			17,911
Foreign currency gains on intercompany loan of a long-term investment nature, net of tax of $0			(19,651)			(19,651)
Unrealized holding gains on available-for-sale debt securities, net of tax				(586)		(586)
Share-based compensation expense		1,448				1,448
Balance at the end of the period at Mar. 31, 2020	$ 1,139	678,319	(9,042)	(548)	(483,831)	186,037
Balance at the end of the period, shares at Mar. 31, 2020	780,514,340
Balance at the beginning of the period at Dec. 31, 2019	$ 943	585,623	(7,302)	38	(455,664)	$ 123,638
Balance at the beginning of the period, shares at Dec. 31, 2019	631,003,568					631,003,568
Increase (Decrease) in Stockholders' Equity
Net loss						$ (58,047)
Other comprehensive loss before reclassifications
Foreign currency translation adjustments						19,105
Foreign currency gains on intercompany loan of a long-term investment nature, net of tax of $0						(20,759)
Unrealized holding gains on available-for-sale debt securities, net of tax						113
Balance at the end of the period at Jun. 30, 2020	$ 1,324	928,777	(8,956)	151	(513,711)	$ 407,585
Balance at the end of the period, shares at Jun. 30, 2020	927,668,946					927,668,946
Balance at the beginning of the period at Mar. 31, 2020	$ 1,139	678,319	(9,042)	(548)	(483,831)	$ 186,037
Balance at the beginning of the period, shares at Mar. 31, 2020	780,514,340
Increase (Decrease) in Stockholders' Equity
Net loss					(29,880)	(29,880)
Issuance of shares upon exercise of stock options	$ 7	4,174				4,181
Issuance of shares upon exercise of stock options (in shares)	5,704,606
Issuance of shares upon completion of public offering, net of issuance costs	$ 178	243,660				243,838
Issuance of shares upon completion of public offering, net of issuance costs (in shares)	141,450,000
Other comprehensive loss before reclassifications
Foreign currency translation adjustments			1,194			1,194
Foreign currency gains on intercompany loan of a long-term investment nature, net of tax of $0			(1,108)			(1,108)
Unrealized holding gains on available-for-sale debt securities, net of tax				699		699
Share-based compensation expense		2,624				2,624
Balance at the end of the period at Jun. 30, 2020	$ 1,324	$ 928,777	$ (8,956)	$ 151	$ (513,711)	$ 407,585
Balance at the end of the period, shares at Jun. 30, 2020	927,668,946					927,668,946