Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit	Total
Balance at Dec. 31, 2019	$ 8	$ 976,955	$ 919	$ (699,171)	$ 278,711
Balance (in shares) at Dec. 31, 2019	78,005
Issuance of common stock upon exercise of stock options		10,808			10,808
Issuance of common stock upon exercise of stock options (in shares)	1,037
Issuance of common stock under employee stock purchase plan		3,062			3,062
Issuance of common stock under employee stock purchase plan (in shares)	97
Equity component of Convertible Notes, net		82,873			82,873
Vesting of restricted stock units (in shares)	578
Stock based compensation		19,374			19,374
Unrealized (loss) gain on available-for-sale securities			4,851		4,851
Cumulative-effect adjustment upon adoption				(403)	(403)
Net loss				(95,009)	(95,009)
Balance at Jun. 30, 2020	$ 8	1,093,072	5,770	(794,583)	304,267
Balance (in shares) at Jun. 30, 2020	79,717
Balance at Mar. 31, 2020	$ 8	988,199	5,666	(734,946)	258,927
Balance (in shares) at Mar. 31, 2020	78,652
Issuance of common stock upon exercise of stock options		6,981			6,981
Issuance of common stock upon exercise of stock options (in shares)	680
Issuance of common stock under employee stock purchase plan		3,062			3,062
Issuance of common stock under employee stock purchase plan (in shares)	97
Equity component of Convertible Notes, net		82,873			82,873
Vesting of restricted stock units (in shares)	288
Stock based compensation		11,957			11,957
Unrealized (loss) gain on available-for-sale securities			104		104
Net loss				(59,637)	(59,637)
Balance at Jun. 30, 2020	$ 8	1,093,072	5,770	(794,583)	304,267
Balance (in shares) at Jun. 30, 2020	79,717
Balance at Dec. 31, 2020	$ 9	1,411,286	4,259	(929,318)	$ 486,236
Balance (in shares) at Dec. 31, 2020	86,223				86,223
Issuance of common stock upon exercise of stock options		6,829			$ 6,829
Issuance of common stock upon exercise of stock options (in shares)	782
Issuance of common stock under employee stock purchase plan		6,085			6,085
Issuance of common stock under employee stock purchase plan (in shares)	106
Vesting of restricted stock units (in shares)	1,387
Stock based compensation		58,335			58,335
Unrealized (loss) gain on available-for-sale securities			(1,818)		(1,818)
Cumulative-effect adjustment upon adoption		(82,876)		6,198	(76,678)
Net loss				(179,879)	(179,879)
Balance at Jun. 30, 2021	$ 9	1,399,659	2,441	(1,102,999)	$ 299,110
Balance (in shares) at Jun. 30, 2021	88,498				88,498
Balance at Mar. 31, 2021	$ 9	1,356,212	3,197	(986,973)	$ 372,445
Balance (in shares) at Mar. 31, 2021	87,430
Issuance of common stock upon exercise of stock options		2,259			2,259
Issuance of common stock upon exercise of stock options (in shares)	207
Issuance of common stock under employee stock purchase plan		6,085			6,085
Issuance of common stock under employee stock purchase plan (in shares)	106
Vesting of restricted stock units (in shares)	755
Stock based compensation		35,103			35,103
Unrealized (loss) gain on available-for-sale securities			(756)		(756)
Net loss				(116,026)	(116,026)
Balance at Jun. 30, 2021	$ 9	$ 1,399,659	$ 2,441	$ (1,102,999)	$ 299,110
Balance (in shares) at Jun. 30, 2021	88,498				88,498