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, 2018		106,162
Balance, value at Dec. 31, 2018	$ (26,405)	$ 106	$ 1,447,954	$ (1,471,577)	$ (12)	$ (2,876)
Stock-based compensation	3,411	$ 0	3,411	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		517
Issuance of common stock under Equity Incentive Plans, value	0	$ 0	0	0	0	0
Repurchase of common stock	(941)	0	0	0	0	(941)
Net loss		0	0	(21,797)	0	0
Unrealized gain on investments	45	$ 0	0	0	45	0
Balance, shares at Mar. 31, 2019		106,679
Balance, value at Mar. 31, 2019	(45,687)	$ 106	1,451,365	(1,493,374)	33	(3,817)
Balance, shares at Dec. 31, 2018		106,162
Balance, value at Dec. 31, 2018	(26,405)	$ 106	1,447,954	(1,471,577)	(12)	(2,876)
Net loss	181,271
Unrealized gain on investments	$ 48
Balance, shares at Sep. 30, 2019	106,679	106,679
Balance, value at Sep. 30, 2019	$ 164,712	$ 106	1,458,693	(1,290,306)	36	(3,817)
Balance, shares at Mar. 31, 2019		106,679
Balance, value at Mar. 31, 2019	(45,687)	$ 106	1,451,365	(1,493,374)	33	(3,817)
Stock-based compensation	3,766	0	3,766	0	0	0
Net loss		0	0	(23,018)	0	0
Unrealized gain on investments	53	$ 0	0	0	53	0
Balance, shares at Jun. 30, 2019		106,679
Balance, value at Jun. 30, 2019	(64,886)	$ 106	1,455,131	(1,516,392)	86	(3,817)
Stock-based compensation	3,562	0	3,562	0	0	0
Net loss	226,086	0	0	226,086	0	0
Unrealized gain on investments	$ (50)	$ 0	0	0	(50)	0
Balance, shares at Sep. 30, 2019	106,679	106,679
Balance, value at Sep. 30, 2019	$ 164,712	$ 106	1,458,693	(1,290,306)	36	(3,817)
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
Issuance of common stock under Equity Incentive Plans, shares		1,032
Issuance of common stock under Equity Incentive Plans, value	2	$ 2	0	0	0	0
Repurchase of common stock	(923)	0	0	0	0	(923)
Net loss	(66,611)	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)
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	(53,079)
Unrealized gain on investments	$ (49)
Balance, shares at Sep. 30, 2020	117,231	117,231
Balance, value at Sep. 30, 2020	$ 87,639	$ 117	1,486,853	(1,394,523)	35	(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)
Stock-based compensation	$ 1,904	0	1,904	0	0	0
Issuance of common stock under Equity Incentive Plans, shares	9,333
Issuance of common stock under Equity Incentive Plans, value	$ 14,096	9	14,087	0	0	0
Net loss	82,603	0	0	82,603	0	0
Unrealized gain on investments	$ (277)	$ 0	0	0	(277)	0
Balance, shares at Sep. 30, 2020	117,231	117,231
Balance, value at Sep. 30, 2020	$ 87,639	$ 117	$ 1,486,853	$ (1,394,523)	$ 35	$ (4,843)