Consolidated Statements of Stockholders' Equity/Deficit-Unaudited - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Gain (Loss)	Treasury Stock
Balance, shares at Dec. 31, 2019		106,679
Balance, value at Dec. 31, 2019	$ 117,101	$ 106	$ 1,462,172	$ (1,341,444)	$ 84	$ (3,817)
Stock-based compensation	4,432	0	4,432	0	0	0
Repurchase of common stock	(923)	0	0	0	0	(923)
Net loss		0	0	(66,611)	0	0
Unrealized gain on investments	776	$ 0	0	0	776	0
Balance, shares at Mar. 31, 2020		107,711
Balance, value at Mar. 31, 2020	54,777	$ 108	1,466,604	(1,408,055)	860	(4,740)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures		1,032
Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture	2	$ 2	0	0	0	0
Balance, shares at Dec. 31, 2019		106,679
Balance, value at Dec. 31, 2019	117,101	$ 106	1,462,172	(1,341,444)	84	(3,817)
Net loss	(135,682)
Unrealized gain on investments	228
Balance, shares at Jun. 30, 2020		107,898
Balance, value at Jun. 30, 2020	(10,687)	$ 108	1,470,862	(1,477,126)	312	(4,843)
Balance, shares at Mar. 31, 2020		107,711
Balance, value at Mar. 31, 2020	54,777	$ 108	1,466,604	(1,408,055)	860	(4,740)
Stock-based compensation	4,258	$ 0	4,258	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		187
Issuance of common stock under Equity Incentive Plans, value	0	$ 0	0	0	0	0
Repurchase of common stock	(103)	0	0	0	0	(103)
Net loss	(69,071)	0	0	(69,071)	0	0
Unrealized gain on investments	(548)	$ 0	0	0	(548)	0
Balance, shares at Jun. 30, 2020		107,898
Balance, value at Jun. 30, 2020	$ (10,687)	$ 108	1,470,862	(1,477,126)	312	(4,843)
Balance, shares at Dec. 31, 2020	142,289	142,289
Balance, value at Dec. 31, 2020	$ 156,371	$ 142	1,561,096	(1,400,018)	(6)	(4,843)
Stock-based compensation	2,851	$ 0	2,851	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		2,000
Issuance of common stock under Equity Incentive Plans, value	16,399	$ 2	16,397	0	0	0
Repurchase of common stock	(2,675)	0	0	0	0	(2,675)
Net loss	(20,958)	0	0	(20,958)	0	0
Unrealized gain on investments	11	$ 0	0	0	11	0
Balance, shares at Mar. 31, 2021		145,552
Balance, value at Mar. 31, 2021	152,547	$ 145	1,580,891	(1,420,976)	5	(7,518)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures		1,263
Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture	$ 548	$ 1	547	0	0	0
Balance, shares at Dec. 31, 2020	142,289	142,289
Balance, value at Dec. 31, 2020	$ 156,371	$ 142	1,561,096	(1,400,018)	(6)	(4,843)
Net loss	(39,025)
Unrealized gain on investments	$ 1
Balance, shares at Jun. 30, 2021	145,640	145,640
Balance, value at Jun. 30, 2021	$ 137,233	$ 146	1,583,653	(1,439,043)	(5)	(7,518)
Balance, shares at Mar. 31, 2021		145,552
Balance, value at Mar. 31, 2021	152,547	$ 145	1,580,891	(1,420,976)	5	(7,518)
Stock-based compensation	$ 2,786	0	2,786	0	0	0
Issuance of common stock under Equity Incentive Plans, shares	88
Issuance of common stock under Equity Incentive Plans, value	$ 8	1	7	0	0	0
Payments of Stock Issuance Costs	31	0	(31)	0	0	0
Net loss	(18,067)	0	0	(18,067)	0	0
Unrealized gain on investments	$ (10)	$ 0	0	0	(10)	0
Balance, shares at Jun. 30, 2021	145,640	145,640
Balance, value at Jun. 30, 2021	$ 137,233	$ 146	$ 1,583,653	$ (1,439,043)	$ (5)	$ (7,518)