Consolidated Statements of Stockholders' Equity (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, 2017		105,711
Balance, value at Dec. 31, 2017	$ 68,265	$ 106	$ 1,435,526	$ (1,365,241)	$ (222)	$ (1,904)
Stock-based compensation	3,074	$ 0	3,074	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		337
Issuance of common stock under Equity Incentive Plans, value	25	$ 0	25	0	0	0
Repurchase of common stock	(972)	0	0	0	0	(972)
Net loss		0	0	(41,820)	0	0
Unrealized gain on investments	(172)	$ 0	0	0	(172)	0
Balance, shares at Mar. 31, 2018		106,048
Balance, value at Mar. 31, 2018	42,612	$ 106	1,438,625	(1,392,849)	(394)	(2,876)
Balance, shares at Dec. 31, 2017		105,711
Balance, value at Dec. 31, 2017	68,265	$ 106	1,435,526	(1,365,241)	(222)	(1,904)
Cumulative effect from accounting change	14,212		0	14,212	0	0
Net loss	(76,369)
Unrealized gain on investments	4
Balance, shares at Jun. 30, 2018		106,115
Balance, value at Jun. 30, 2018	11,504	$ 106	1,441,890	(1,427,398)	(218)	(2,876)
Balance, shares at Mar. 31, 2018		106,048
Balance, value at Mar. 31, 2018	42,612	$ 106	1,438,625	(1,392,849)	(394)	(2,876)
Stock-based compensation	2,898	$ 0	2,898	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		67
Issuance of common stock under Equity Incentive Plans, value	367	$ 0	367	0	0	0
Net loss	(34,549)	0	0	(34,549)	0	0
Unrealized gain on investments	176	$ 0	0	0	176	0
Balance, shares at Jun. 30, 2018		106,115
Balance, value at Jun. 30, 2018	11,504	$ 106	1,441,890	(1,427,398)	(218)	(2,876)
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	(44,815)
Unrealized gain on investments	98
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)
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	(23,018)	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)