Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment
Beginning balance (in shares) at Dec. 31, 2018			54,738,000
Beginning balance at Dec. 31, 2018	$ 1,885,070	$ 6,564	$ 547	$ 3,386,958	$ (3,627)	$ (1,498,808)	$ 6,564
Vesting of restricted stock units (in shares)			251,000
Vesting of restricted stock units	0		$ 3	(3)
Exercise of stock options (in shares)			354,000
Exercise of stock options	17,838		$ 4	17,834
Purchase of common stock under ESPP (in shares)			25,000
Purchase of common stock under ESPP	2,766			2,766
Stock-based compensation	160,629			160,629
Other comprehensive income (loss)	1,734				1,734
Net loss	(789,608)					(789,608)
Ending balance (in shares) at Dec. 31, 2019			55,368,000
Ending balance at Dec. 31, 2019	$ 1,284,993		$ 554	3,568,184	(1,893)	(2,281,852)
Accounting Standards Update [Extensible Enumeration]	Accounting Standards Update 2016-02 [Member]
Issuance of common stock (in shares)			10,455,000
Issuance of common stock	$ 541,536		$ 105	541,431
Vesting of restricted stock units (in shares)			434,000
Vesting of restricted stock units	0		$ 4	(4)
Exercise of stock options (in shares)			95,000
Exercise of stock options	1,847		$ 1	1,846
Purchase of common stock under ESPP (in shares)			80,000
Purchase of common stock under ESPP	3,775		$ 1	3,774
Stock-based compensation	145,212			145,212
Other comprehensive income (loss)	(3,612)				(3,612)
Net loss	$ (618,695)					(618,695)
Ending balance (in shares) at Dec. 31, 2020	66,432,000		66,432,000
Ending balance at Dec. 31, 2020	$ 1,355,056		$ 665	4,260,443	(5,505)	(2,900,547)
Issuance of common stock (in shares)			2,273,000
Issuance of common stock	37,500		$ 23	37,477
Vesting of restricted stock units (in shares)			534,000
Vesting of restricted stock units	$ 0		$ 5	(5)
Exercise of stock options (in shares)	218,000		218,000
Exercise of stock options	$ 1,488		$ 2	1,486
Purchase of common stock under ESPP (in shares)			120,000
Purchase of common stock under ESPP	2,581		$ 1	2,580
Issuance of pre-funded warrants	37,477			37,477
Issuance of unrestricted stock awards to settle accrued employee compensation (in shares)			1,538,000
Issuance of unrestricted stock awards to settle accrued employee compensation	25,074		$ 15	25,059
Stock-based compensation	110,260			110,260
Distribution of 2seventy bio	378,145			378,375	(230)
Other comprehensive income (loss)	2,364				2,364
Net loss	$ (819,378)					(819,378)
Ending balance (in shares) at Dec. 31, 2021	71,115,000		71,115,000
Ending balance at Dec. 31, 2021	$ 374,277		$ 711	$ 4,096,402	$ (2,911)	$ (3,719,925)