Condensed Consolidated Statements of Cash Flows $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)
Cash flows from operating activities:
Net loss	$ (353,046)	$ (144,630)
Adjustments to reconcile net loss to cash flows from operating activities:
Acquired in-process research and development	173,240
Depreciation and amortization	14,064	5,125
Amortization of investment discount	(5,800)	(2,828)
Non-cash interest expense	10,513	9,958
Stock-based compensation	35,901	25,805
Other	892	37
Changes in operating assets and liabilities, net:
Net increase in accounts receivable	(7,937)	(13,517)
Net increase in inventory	(31,124)	(20,521)
Net increase in other assets	(87,708)	(29,866)
Net increase in accounts payable, accrued expenses, deferred revenue and other liabilities	25,768	17,515
Net cash used in operating activities	(225,237)	(152,922)
Cash flows from investing activities:
Purchase of property and equipment	(32,945)	(20,863)
Purchase of intangible assets	(1,454)	(1,556)
Purchase of available-for-sale securities	(731,288)	(295,823)
Purchases of restricted investment		(353)
Maturity and sale of available-for-sale securities	1,225,847	249,243
Net cash provided by (used in) investing activities	287,604	(69,352)
Cash flows from financing activities:
Proceeds from revolving line of credit		173,354
Repayment of revolving line of credit		(173,653)
Repayments on mortgage loans		(1,265)
Proceeds from sale of common stock, net of offering costs	365,353
Taxes paid related to net share settlement of equity awards	(889)
Proceeds from exercise of stock options and purchase of stock under the Employee Stock Purchase Program	19,391	34,386
Net cash provided by financing activities	383,855	32,822
Increase (decrease) in cash, cash equivalents and restricted cash	446,222	(189,452)
Cash, cash equivalents and restricted cash:
Beginning of period	370,829	599,827
End of period	817,051	410,375
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	808,591	$ 410,375
Restricted cash in other assets	$ 8,460
Restricted Cash, Noncurrent, Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Total cash, cash equivalents and restricted cash	$ 817,051	$ 410,375
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	4,275	6,058
Supplemental schedule of non-cash investing activities and financing activities:
Sale of available-for-sale securities included in investment receivable	20,000
Reclassification of long term investments to short term investments		9,980
Intangible assets included in accrued expenses	226	294
Accrual for debt issuance costs related to the term loans		600
Property and equipment included in accrued expenses	2,902	$ 289
Myonexus Therapeutics, Inc. [Member]
Cash flows from investing activities:
Acquisition of Myonexus Therapeutics, Inc., net of cash acquired	$ (172,556)