Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (789,608)	$ (555,625)	$ (335,643)
Adjustments to reconcile net loss to net cash used in operating    activities:
Change in fair value of contingent consideration	2,747	2,999	(2,189)
Depreciation and amortization	17,434	17,158	13,538
Stock-based compensation expense	160,629	110,836	53,282
Unrealized loss (gain) on equity securities	(9,297)	2,154	0
Other non-cash items	(11,000)	(5,880)	3,153
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(13,913)	(24,288)	(20,092)
Operating lease right-of-use assets	22,496
Accounts payable	23,600	3,614	526
Accrued expenses and other liabilities	46,291	37,832	5,848
Operating lease liabilities	(9,944)
Deferred revenue	(16,674)	(41,872)	1,024
Collaboration research advancement	(5,739)	43,954	0
Net cash used in operating activities	(564,384)	(413,426)	(280,553)
Cash flows from investing activities:
Purchase of property, plant and equipment	(71,028)	(55,737)	(62,242)
Purchases of marketable securities	(756,570)	(1,517,982)	(686,204)
Proceeds from maturities of marketable securities	1,340,629	894,284	431,816
Purchase of intangible assets	(5,224)	0	0
Net cash provided by (used in) investing activities	507,807	(679,435)	(316,630)
Cash flows from financing activities:
Cash paid for contingent purchase price consideration	0	0	(1,074)
Reimbursement of assets under financing lease obligation	0	3,098	38,021
Payments on financing lease obligation		(1,017)	(574)
Proceeds from issuance of common stock	0	649,368	1,006,570
Proceeds from exercise of stock options and ESPP contributions	21,187	31,759	33,231
Net cash provided by financing activities	21,187	737,692	1,076,174
(Decrease) increase in cash, cash equivalents and restricted cash	(35,390)	(355,169)	478,991
Cash, cash equivalents and restricted cash at beginning of year	417,099	772,268	293,277
Cash, cash equivalents and restricted cash at end of year	381,709	417,099	772,268
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	327,214	402,579	758,505
Restricted cash included in receivables and other current assets	0	364	0
Restricted cash included in restricted cash and other non-current assets	54,495	14,156	13,763
Supplemental cash flow disclosures:
Assets acquired under financing lease obligation	0	0	3,271
Purchases of property, plant and equipment included in accounts    payable and accrued expenses	5,286	7,449	2,566
Right-of-use assets obtained in exchange for operating lease liabilities	23,939
Cash paid during the period for interest	0	15,494	11,411
Cash paid during the period for income taxes	637	358	91
Regeneron Collaboration Agreement
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 0	$ 54,484	$ 0