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)
Cumulative effect from accounting change	14,212
Stock-based compensation	3,074
Issuance of common stock under Equity Incentive Plans, value	25
Net income (loss)				(41,820)
Unrealized gain (loss) on investments	(172)
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)
Stock-based compensation		$ 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		$ 0	25	0	0	0
Repurchase of common stock	(972)	0	0	0	0	(972)
Net income (loss)		0	0	(41,820)	0	0
Unrealized gain (loss) on investments		$ 0	0	0	(172)	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, 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
Net income (loss)	(103,765)
Unrealized gain (loss) on investments	151
Balance, shares at Sep. 30, 2018		106,134
Balance, value at Sep. 30, 2018	(12,710)	$ 106	1,444,925	(1,454,794)	(71)	(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 income (loss)		0	0	(34,549)	0	0
Unrealized gain (loss) 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)
Stock-based compensation	2,877	$ 0	2,877	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		19
Issuance of common stock under Equity Incentive Plans, value	158	$ 0	158	0	0	0
Net income (loss)	(27,396)	0	0	(27,396)	0	0
Unrealized gain (loss) on investments	147	$ 0	0	0	147	0
Balance, shares at Sep. 30, 2018		106,134
Balance, value at Sep. 30, 2018	(12,710)	$ 106	1,444,925	(1,454,794)	(71)	(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)
Net income (loss)	181,271
Unrealized gain (loss) on investments	48
Balance, shares at Sep. 30, 2019		106,679
Balance, value at Sep. 30, 2019	$ 164,712	$ 106	$ 1,458,693	$ (1,290,306)	$ 36	$ (3,817)