Restatement and Revision of Quarterly Condensed Consolidated Financial Statements - Consolidated Statements of Stockholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended								6 Months Ended	9 Months Ended			12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Error corrections and prior period adjustments restatement
Balance	$ 965,828	$ 972,730	$ 971,597	$ 975,373	$ 963,082	$ 973,226		$ 1,012,112	$ 975,373	$ 975,373	$ 1,012,112	$ 571,584	$ 975,373	$ 1,012,112	$ 571,584
Increase (Decrease) in Stockholders' Equity
Net income (loss)	$ (12,055)	(12,737)	(9,140)	(6,539)	(6,876)	(15,532)	$ (36,980)	$ (26,576)	(15,679)	(28,416)	(79,088)	$ (155,939)	(40,471)	(85,964)	(157,957)
Foreign currency translation adjustment		6	2	0		(2,096)			2	8	(2,270)		24	10,145	(4,447)
Unrealized gain (loss) on marketable securities, net of tax		(86)	134	(96)		17			38	(48)	101		(13)	54	(747)
Issuance of common stock upon exercise of options, equity stock and employee stock purchase plan		(919)	3,430	(3,213)		79			217	(702)	6,101		2,084	8,081	4,630
Repurchase and retirement of common stock under repurchase plan, shares	(2,600,000)
Repurchase and retirement of common stock under repurchase plan, value	$ (40,000)												(50,069)		(50,033)
Stock-based compensation		6,834	6,707	6,072		7,388			12,779	19,613	21,657		25,778	26,476	21,736
Balance	$ 912,706	$ 965,828	$ 972,730	971,597	975,373	963,082	$ 973,226		972,730	965,828	963,082		912,706	975,373	$ 1,012,112
Cumulative effect, period of adoption, adjustment | Accounting Standards Update 2016-02
Error corrections and prior period adjustments restatement
Balance				$ 4,469	4,469				$ 4,469	$ 4,469			$ 4,469
Increase (Decrease) in Stockholders' Equity
Balance					$ 4,469	$ 4,469					$ 4,469			$ 4,469
Common stock
Error corrections and prior period adjustments restatement
Balance (in shares)	113,923,000	113,744,000	113,275,000	112,131,000	111,493,000	111,127,000		109,018,000	112,131,000	112,131,000	109,018,000	109,764,000	112,131,000	109,018,000	109,764,000
Balance	$ 114	$ 114	$ 113	$ 112	$ 111	$ 111		$ 109	$ 112	$ 112	$ 109	$ 110	$ 112	$ 109	$ 110
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options, equity stock and stock units, and employee stock purchase plan (in shares)		179,000	469,000	1,144,000		366,000			1,613,000	1,792,000	2,475,000		2,183,000	3,113,000	2,616,000
Issuance of common stock upon exercise of options, equity stock and employee stock purchase plan		$ 0	$ 1	$ 1		$ 0			$ 2	$ 2	$ 2		$ 3	$ 3	$ 3
Repurchase and retirement of common stock under repurchase plan, shares													(2,616,000)		(3,786,000)
Repurchase and retirement of common stock under repurchase plan, value													$ (3)		$ (4)
Issuance of common stock in connection with the maturity of the 2018 Notes related to the settlement of the in-the-money conversion feature of the 2018 Notes															424,000
Balance (in shares)	111,698,000	113,923,000	113,744,000	113,275,000	112,131,000	111,493,000	111,127,000		113,744,000	113,923,000	111,493,000		111,698,000	112,131,000	109,018,000
Balance	$ 112	$ 114	$ 114	$ 113	$ 112	$ 111	$ 111		$ 114	$ 114	$ 111		$ 112	$ 112	$ 109
Additional paid-in capital
Error corrections and prior period adjustments restatement
Balance	1,280,051	1,274,136	1,264,000	1,261,142	1,254,344	1,246,877		1,226,588	1,261,142	1,261,142	1,226,588	1,212,798	1,261,142	1,226,588	1,212,798
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options, equity stock and employee stock purchase plan		(919)	3,429	(3,214)		79			215	(704)	6,099		2,081	8,078	4,627
Repurchase and retirement of common stock under repurchase plan, value													(18,575)		(12,573)
Stock-based compensation		6,834	6,707	6,072		7,388			12,779	19,613	21,657		25,778	26,476	21,736
Balance	1,270,426	1,280,051	1,274,136	1,264,000	1,261,142	1,254,344	1,246,877		1,274,136	1,280,051	1,254,344		1,270,426	1,261,142	1,226,588
Accumulated deficit
Error corrections and prior period adjustments restatement
Balance	(314,205)	(301,468)	(292,328)	(285,789)	(278,913)	(263,381)		(204,294)	(285,789)	(285,789)	(204,294)	(636,227)	(285,789)	(204,294)	(636,227)
Increase (Decrease) in Stockholders' Equity
Net income (loss)		(12,737)	(9,140)	(6,539)		(15,532)			(15,679)	(28,416)	(79,088)		(40,471)	(85,964)	(157,957)
Repurchase and retirement of common stock under repurchase plan, value													(31,491)		(37,456)
Balance	(357,751)	(314,205)	(301,468)	(292,328)	(285,789)	(278,913)	(263,381)		(301,468)	(314,205)	(278,913)		(357,751)	(285,789)	(204,294)
Accumulated deficit | Cumulative effect, period of adoption, adjustment | Accounting Standards Update 2016-02
Error corrections and prior period adjustments restatement
Balance				4,469	4,469				4,469	4,469			4,469
Increase (Decrease) in Stockholders' Equity
Balance					4,469	4,469					4,469			4,469
Accumulated other comprehensive gain (loss)
Error corrections and prior period adjustments restatement
Balance	(132)	(52)	(188)	(92)	(12,460)	(10,381)		$ (10,291)	(92)	(92)	(10,291)	$ (5,097)	(92)	(10,291)	(5,097)
Increase (Decrease) in Stockholders' Equity
Foreign currency translation adjustment		6	2			(2,096)			2	8	(2,270)		24	10,145	(4,447)
Unrealized gain (loss) on marketable securities, net of tax		(86)	134	(96)		17			38	(48)	101		(13)	54	(747)
Balance	$ (81)	$ (132)	$ (52)	$ (188)	$ (92)	$ (12,460)	$ (10,381)		$ (52)	$ (132)	$ (12,460)		$ (81)	$ (92)	$ (10,291)