Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (202,611)	$ (164,446)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	(3,108)	296
Depreciation and amortization	4,880	3,783
Stock-based compensation expense	36,293	32,341
Unrealized loss on equity securities	4,520	3,085
Other non-cash items	(1,387)	(3,456)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(10,984)	(12,793)
Operating lease right-of-use assets	5,842	5,374
Accounts payable	(9,519)	10,590
Accrued expenses and other liabilities	(20,557)	(21,514)
Operating lease liabilities	(5,070)	3,224
Deferred revenue	(2,118)	(8,672)
Collaboration research advancement	(2,302)	(1,966)
Net cash used in operating activities	(206,121)	(154,154)
Cash flows from investing activities:
Purchase of property, plant and equipment	(10,676)	(19,321)
Purchases of marketable securities	(101,421)	(381,735)
Proceeds from maturities of marketable securities	336,675	364,143
Net cash provided by (used in) investing activities	224,578	(36,913)
Cash flows from financing activities:
Proceeds from exercise of stock options and ESPP contributions	963	10,223
Net cash provided by financing activities	963	10,223
Increase (decrease) in cash, cash equivalents and restricted cash	19,420	(180,844)
Cash, cash equivalents and restricted cash at beginning of period	381,709	417,099
Cash, cash equivalents and restricted cash at end of period	401,129	236,255
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents	346,629	221,738
Restricted cash included in receivables and other current assets	0	100
Restricted cash included in restricted cash and other non-current assets	54,500	14,417
Cash, cash equivalents and restricted cash at end of period	401,129	236,255
Supplemental cash flow disclosures from investing and financing activities:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	1,125	9,302
Right-of-use assets obtained in exchange for operating lease liabilities	14,425	5,500
Tenant improvements included in receivables and other current assets	$ 1,516	$ 8,009