Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss		$ (554,128)	$ (715,075)	$ (361,918)
Adjustments to reconcile net loss to cash flows in operating activities:
Loss on contingent consideration	[1]	45,000
Gain from sale of Priority Review Voucher		(108,069)
Acquired in-process research and development			173,240
Non-cash up-front payment to StrideBio			29,415
Depreciation and amortization		26,911	24,500	12,245
Reduction in the carry amounts of the right of use assets		12,828	6,047
Amortization of investment discount		(2,186)	(8,445)	(7,672)
Loss from debt extinguishment				2,322
Non-cash interest expense		25,454	21,444	20,190
Stock-based compensation		108,070	78,602	50,127
Other		(470)	690	3,938
Changes in operating assets and liabilities, net:
Net increase in accounts receivable		(10,461)	(41,835)	(19,576)
Net increase in inventory		(60,582)	(45,934)	(41,840)
Net increase in other assets		(166,328)	(102,091)	(136,638)
Net increase in deferred revenue		749,429
Net increase in accounts payable, accrued expenses, lease liabilities and other liabilities		41,998	122,979	90,162
Net cash provided by (used in) operating activities		107,466	(456,463)	(388,660)
Cash flows from investing activities:
Purchase of property and equipment		(82,202)	(59,631)	(61,157)
Proceeds from sale of Priority Review Voucher		108,069
Purchase of available-for-sale securities		(1,333,568)	(1,193,632)	(1,171,603)
Maturity and sales of available-for-sale securities		1,189,480	1,715,626	865,813
Acquisition of Myonexus Therapeutics, Inc., net of cash acquired			(172,556)
Other		(3,500)	(3,082)	(3,541)
Net cash (used in) provided by investing activities		(121,721)	286,725	(370,488)
Cash flows from financing activities:
Proceeds from issuance of common stock to Roche, net of offering costs		312,053	365,354	513,409
Proceeds from exercise of stock options and purchase of stock under the Employee Stock Purchase Program		83,957	36,601	50,222
Taxes paid related to net share settlement of equity awards		(4,798)	(4,337)
Proceeds from term loans		291,150	245,625
Repayment of July 2017 Term Loan				(30,000)
Proceeds from revolving line of credit				235,872
Repayment of revolving line of credit				(235,954)
Payment for debt extinguishment				(2,134)
Repayment of mortgage loans and notes payable				(1,265)
Debt issuance costs		(39)	(689)
Net cash provided by financing activities		682,323	642,554	530,150
Increase (decrease) in cash and cash equivalents		668,068	472,816	(228,998)
Cash, cash equivalents and restricted cash:
Beginning of year		843,645	370,829	599,827
End of year		1,511,713	843,645	370,829
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents		1,502,648	835,080	$ 370,829
Restricted cash in other assets		$ 9,065	$ 8,565
Restricted Cash, Noncurrent, Asset, Statement of Financial Position [Extensible List]		us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Total cash, cash equivalents and restricted cash		$ 1,511,713	$ 843,645	$ 370,829
Supplemental disclosure of cash flow information:
Cash paid during the period for interest		34,418	8,550	11,308
Cash paid during the period for income taxes		2,510	933	1,548
Lease liabilities arising from obtaining right of use assets		59,327
Intangible assets and property and equipment included in accrued expenses		5,151	1,309	5,795
Shares withheld for tax included in accrued expenses		6,333	4,798
Accrued debt issuance costs		$ 11,000	$ 5,000
Reclassification of long term investments to short term investments				$ 9,980

[1] The loss on contingent consideration is related to the fair value adjustment of the regulatory-related contingent payments that are accounted for as derivatives. Please see Note 5. Fair Value Measurements for further details.