Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Cash flows from operating activities:
Net loss	$ (618,695)	$ (789,608)	$ (555,625)
Adjustments to reconcile net loss to net cash used in operating    activities:
Change in fair value of contingent consideration	(6,468)	2,747	2,999
Depreciation and amortization	19,356	17,434	17,158
Stock-based compensation expense	156,631	160,629	110,836
Unrealized loss (gain) on equity securities	7,217	9,297	(2,154)
Other non-cash items	458	(11,000)	(5,880)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(10,089)	(13,913)	(24,288)
Operating lease right-of-use assets	21,281	22,496	0
Accounts payable	(20,100)	23,600	3,614
Accrued expenses and other liabilities	(4,982)	46,291	37,832
Operating lease liabilities	(17,380)	(9,944)	0
Deferred revenue	9,817	(16,674)	(41,872)
Collaboration research advancement	(7,397)	(5,739)	43,954
Net cash used in operating activities	(470,351)	(564,384)	(413,426)
Cash flows from investing activities:
Purchase of property, plant and equipment	(28,986)	(71,028)	(55,737)
Purchases of marketable securities	(1,003,525)	(756,570)	(1,517,982)
Sales of marketable securities	29,878	0	0
Proceeds from maturities of marketable securities	918,288	1,340,629	894,284
Purchase of intangible assets	0	(5,224)	0
Net cash provided by (used in) investing activities	(84,345)	507,807	(679,435)
Cash flows from financing activities:
Reimbursement of assets under financing lease obligation	0	0	3,098
Payments on financing lease obligation	0	0	(1,017)
Proceeds from exercise of stock options and ESPP contributions	5,179	21,187	31,759
Net cash provided by financing activities	546,715	21,187	737,692
Decrease in cash, cash equivalents and restricted cash	(7,981)	(35,390)	(355,169)
Cash, cash equivalents and restricted cash at beginning of year	381,709	417,099	772,268
Cash, cash equivalents and restricted cash at end of year	373,728	381,709	417,099
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	317,705	327,214	402,579
Restricted cash included in receivables and other current assets	1,500	0	364
Restricted cash included in restricted cash and other non-current assets	54,523	54,495	14,156
Supplemental cash flow disclosures:
Purchases of property, plant and equipment included in accounts    payable and accrued expenses	2,854	5,286	7,449
Right-of-use assets obtained in exchange for operating lease liabilities	19,414	23,939	0
Cash paid during the period for interest	0	0	15,494
Cash paid during the period for income taxes	361	637	358
Public Offering
Cash flows from financing activities:
Proceeds from issuance of common stock	541,536	0	649,368
Regeneron Collaborative Arrangement
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 0	$ 0	$ 54,484