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	$ (715,075)	$ (361,918)	$ (50,688)
Adjustments to reconcile net loss to cash flows in operating activities:
Acquired in-process research and development	173,240
Non-cash up-front payment to StrideBio	29,415
Gain from sale of Priority Review Voucher			(125,000)
Depreciation and amortization	30,547	12,245	8,092
Amortization of investment discount	(8,445)	(7,672)	(888)
Loss from debt extinguishment		2,322
Non-cash interest expense	21,444	20,190	2,679
Stock-based compensation	78,602	50,127	30,465
Other	690	3,938	805
Changes in operating assets and liabilities, net:
Net increase in accounts receivable	(41,835)	(19,576)	(24,240)
Net increase in inventory	(45,934)	(41,840)	(70,792)
Net increase in other assets	(102,091)	(136,638)	(15,354)
Net increase in accounts payable, accrued expenses, lease liabilities and other liabilities	122,979	90,162	12,925
Net cash used in operations	(456,463)	(388,660)	(231,996)
Cash flows from investing activities:
Purchase of property and equipment	(59,631)	(61,157)	(12,000)
Purchase of intangible assets	(3,082)	(3,188)	(9,215)
Purchase of available-for-sale securities	(1,193,632)	(1,171,603)	(589,520)
Maturity and sales of available-for-sale securities	1,715,626	865,813	296,225
Proceeds from sale of Priority Review Voucher			125,000
Purchase of restricted investment		(353)
Maturity of restricted investment			10,695
Acquisition of Myonexus Therapeutics, Inc., net of cash acquired	(172,556)
Net cash provided (used) in investing activities	286,725	(370,488)	(178,815)
Cash flows from financing activities:
Proceeds from exercise of options and employee stock purchase program	36,601	50,222	15,224
Taxes paid related to net share settlement of equity awards	(4,337)
Proceeds from sales of common stock, net of offering costs	365,354	513,409	353,959
Repayment of June 2015 and July 2017 Term Loan		(30,000)	(15,000)
Proceeds from revolving line of credit		235,872	39,708
Repayment of revolving line of credit		(235,954)	(39,645)
Payment for debt extinguishment		(2,134)
Repayment of mortgage loans and notes payable		(1,265)	(109)
Purchase of capped call options			(50,901)
Proceeds from convertible debt offering			570,000
Debt issuance costs	(689)		(15,154)
Net cash provided by financing activities	642,554	530,150	888,082
Increase (decrease) in cash and cash equivalents	472,816	(228,998)	477,271
Cash, cash equivalents and restricted cash:
Beginning of period	370,829	599,827	122,556
End of period	843,645	370,829	599,827
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	835,080	$ 370,829	599,691
Restricted cash in other assets	8,565		$ 136
Restricted Cash, Noncurrent, Asset, Statement of Financial Position [Extensible List]		us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Total cash, cash equivalents and restricted cash	843,645	$ 370,829	$ 599,827
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	8,550	11,308	1,912
Cash paid during the period for income taxes	933	1,548	5,336
Supplemental schedule of non-cash investing activities and financing activities:
Shares withheld for tax included in accrued expenses	4,798
Accrued debt discount and issuance costs	5,000		625
Property and equipment included in accrued expenses	1,181	5,421	2,525
Reclassification of long term investments to short term investments		$ 9,980
December 2019 Term Loan [Member]
Cash flows from financing activities:
Proceeds from Term Loan	$ 245,625
July 2017 Term Loan [Member]
Cash flows from financing activities:
Proceeds from Term Loan			$ 30,000