Condensed Consolidated Statements of Shareholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-In Capital	Additional Paid-In Capital Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment
Beginning balance (in shares) at Dec. 31, 2019			46,189,870
Beginning balance at Dec. 31, 2019	$ (42,279)		$ 5	$ 1,194,327		$ 3	$ (1,236,614)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(87,823)
Unrealized gain (loss) from available-for-sale securities	79					79
Vesting of restricted shares (in shares)			242,974
Vesting of restricted shares	0
Issuance of common stock upon purchase by employee stock purchase plan (in shares)			115,357
Issuance of common stock upon purchase by employee stock purchase plan	1,621			1,621
Share-based compensation expense	19,821			19,821
Ending balance (in shares) at Sep. 30, 2020			46,548,201
Ending balance at Sep. 30, 2020	(108,581)		$ 5	1,215,769		82	(1,324,437)
Beginning balance (in shares) at Dec. 31, 2019			46,189,870
Beginning balance at Dec. 31, 2019	(42,279)		$ 5	1,194,327		3	(1,236,614)
Ending balance (in shares) at Dec. 31, 2020			46,779,479
Ending balance at Dec. 31, 2020	$ (123,659)	$ (86,433)	$ 5	1,222,137	$ (134,450)	21	(1,345,822)	$ 48,017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	Accounting Standards Update 2020-06
Beginning balance (in shares) at Jun. 30, 2020			46,448,491
Beginning balance at Jun. 30, 2020	$ (109,419)		$ 5	1,208,616		108	(1,318,148)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(6,289)						(6,289)
Unrealized gain (loss) from available-for-sale securities	(26)					(26)
Vesting of restricted shares (in shares)			39,650
Vesting of restricted shares	0
Issuance of common stock upon purchase by employee stock purchase plan (in shares)			60,060
Issuance of common stock upon purchase by employee stock purchase plan	631			631
Share-based compensation expense	6,522			6,522
Ending balance (in shares) at Sep. 30, 2020			46,548,201
Ending balance at Sep. 30, 2020	(108,581)		$ 5	1,215,769		82	(1,324,437)
Beginning balance (in shares) at Dec. 31, 2020			46,779,479
Beginning balance at Dec. 31, 2020	(123,659)	$ (86,433)	$ 5	1,222,137	$ (134,450)	21	(1,345,822)	$ 48,017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(54,592)						(54,592)
Unrealized gain (loss) from available-for-sale securities	(21)					(21)
Vesting of restricted shares (in shares)			243,989
Vesting of restricted shares	0
Exercise of options (in shares)			195,758
Exercise of options	3,805			3,805
Issuance of common stock upon purchase by employee stock purchase plan (in shares)			106,767
Issuance of common stock upon purchase by employee stock purchase plan	1,154			1,154
Share-based compensation expense	17,197			17,197
Ending balance (in shares) at Sep. 30, 2021			47,325,993
Ending balance at Sep. 30, 2021	(242,549)		$ 5	1,109,843		0	(1,352,397)
Beginning balance (in shares) at Mar. 31, 2021			47,255,094
Beginning balance at Mar. 31, 2021	(227,077)		$ 5	1,103,282		0	(1,330,364)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(22,033)						(22,033)
Unrealized gain (loss) from available-for-sale securities	0
Vesting of restricted shares (in shares)			30,350
Vesting of restricted shares	0
Exercise of options (in shares)			83
Exercise of options	1			1
Share-based compensation expense	6,083			6,083
Ending balance (in shares) at Jun. 30, 2021			47,325,993
Ending balance at Jun. 30, 2021	(242,549)		$ 5	1,109,843		0	(1,352,397)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(22,033)
Unrealized gain (loss) from available-for-sale securities	0
Issuance of common stock upon purchase by employee stock purchase plan (in shares)			40,466
Issuance of common stock upon purchase by employee stock purchase plan	477			477
Ending balance (in shares) at Sep. 30, 2021			47,325,993
Ending balance at Sep. 30, 2021	$ (242,549)		$ 5	$ 1,109,843		$ 0	$ (1,352,397)