Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Cash flows from operating activities:
Net loss	$ (819,378)	$ (618,695)	$ (789,608)
Adjustments to reconcile net loss to net cash used in operating    activities:
Change in fair value of contingent consideration	387	(6,468)	2,747
Depreciation and amortization	19,649	19,356	17,434
Stock-based compensation expense	127,915	156,631	160,629
Unrealized loss (gain) on equity securities	(29,356)	7,217	9,297
Excess inventory reserve	29,924	0	0
Other non-cash items	17,235	458	(11,000)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(5,289)	(10,089)	(13,913)
Inventory	(18,447)	0	0
Operating lease right-of-use assets	30,148	21,281	22,496
Accounts payable	9,286	(20,100)	23,600
Accrued expenses and other liabilities	40,025	4,835	29,617
Operating lease liabilities	(32,142)	(17,380)	(9,944)
Collaboration research advancement	(5,596)	(7,397)	(5,739)
Net cash used in operating activities	(635,639)	(470,351)	(564,384)
Cash flows from investing activities:
Purchase of property, plant and equipment	(14,503)	(28,986)	(71,028)
Purchases of marketable securities	(451,391)	(1,003,525)	(756,570)
Proceeds from maturities of marketable securities	895,333	918,288	1,340,629
Proceeds from sales of marketable securities	31,318	29,878	0
Proceeds from sale of Durham, North Carolina manufacturing facility	110,300	0	0
Purchase of intangible assets	(8,500)	0	(5,224)
Net cash provided by (used in) investing activities	562,557	(84,345)	507,807
Cash flows from financing activities:
Net cash transferred to 2seventy bio at separation	(174,284)	0	0
Proceeds from exercise of stock options and ESPP contributions	5,355	5,179	21,187
Net cash provided by (used in) financing activities	(93,954)	546,715	21,187
Decrease in cash, cash equivalents and restricted cash	(167,036)	(7,981)	(35,390)
Cash, cash equivalents and restricted cash at beginning of year	373,728	381,709	417,099
Cash, cash equivalents and restricted cash at end of year	206,692	373,728	381,709
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	161,160	317,705	327,214
Restricted cash included in receivables and other current assets	2,282	1,500	0
Restricted cash included in restricted cash and other non-current assets	43,250	54,523	54,495
Total cash, cash equivalents and restricted cash	206,692	373,728	381,709
Supplemental cash flow disclosures:
Purchases of property, plant and equipment included in accounts    payable and accrued expenses	411	2,854	5,286
Right-of-use assets obtained in exchange for operating lease liabilities	202,221	19,414	23,939
Reduction of right of use asset and associated lease liability due to lease reassessment	(9,004)	0	0
Issuance of unrestricted stock awards to settle accrued employee compensation	25,074	0	0
Cash paid during the period for interest	0	0	0
Cash paid during the period for income taxes	617	361	637
Private Placement
Cash flows from financing activities:
Proceeds from issuance of common stock	74,975	0	0
Public Offering
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 0	$ 541,536	$ 0