Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net loss	$ (224,076)	$ (360,228)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	(4,763)	510
Depreciation and amortization	9,430	7,831
Stock-based compensation expense	84,822	87,452
Unrealized loss on equity securities	3,343	6,184
Other non-cash items	(1,841)	(7,064)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(13,813)	(20,694)
Operating lease right-of-use assets	11,085	11,037
Accounts payable	(14,042)	6,652
Accrued expenses and other liabilities	(14,025)	(9,301)
Operating lease liabilities	(10,131)	(259)
Deferred revenue	11,412	(11,716)
Collaboration research advancement	(3,779)	(2,431)
Net cash used in operating activities	(166,378)	(292,027)
Cash flows from investing activities:
Purchase of property, plant and equipment	(15,478)	(37,925)
Purchases of marketable securities	(101,421)	(471,365)
Sales of marketable securities	29,878	0
Proceeds from maturities of marketable securities	580,875	704,803
Net cash provided by investing activities	493,854	195,513
Cash flows from financing activities:
Proceeds from public offering of common stock, net of issuance costs	541,536	0
Proceeds from exercise of stock options and ESPP contributions	2,549	15,004
Net cash provided by financing activities	544,085	15,004
Increase (decrease) in cash, cash equivalents and restricted cash	871,561	(81,510)
Cash, cash equivalents and restricted cash at beginning of period	381,709	417,099
Cash, cash equivalents and restricted cash at end of period	1,253,270	335,589
Reconciliation of cash, cash equivalents and restricted cash:
Total cash, cash equivalents and restricted cash	1,253,270	335,589
Supplemental cash flow disclosures from investing and financing activities:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	1,257	8,869
Right-of-use assets obtained in exchange for operating lease liabilities	$ 14,663	$ 17,489