CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net loss	$ (141,214)	$ (107,290)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,178	3,000
Stock-based compensation	19,584	15,717
Charge for 401(k) company stock match	942	551
Realized loss on sale of marketable equity securities		1,549
Other		608
Changes in operating assets and liabilities:
Billed and unbilled collaboration receivables	(16,766)	3,877
Prepaid expenses and other assets	(27,540)	3,708
Accounts payable	(9,042)	(11,067)
Accrued expenses and other	(11,767)	(10,855)
Deferred revenue	26,961	(128)
Net cash used in operating activities	(155,664)	(100,330)
Cash flows from investing activities:
Purchases of property, plant and equipment	(21,257)	(36,152)
Purchases of marketable securities	(358,433)	(86,803)
Sales and maturities of marketable securities	244,876	198,031
Net cash (used in) provided by investing activities	(134,814)	75,076
Cash flows from financing activities:
Proceeds from exercise of stock options and other types of equity	42,094	2,769
Payments for repurchase of common stock for employee tax withholding	(572)	(53)
Net cash provided by financing activities	41,522	2,716
Net decrease in cash, cash equivalents and restricted cash	(248,956)	(22,538)
Cash, cash equivalents and restricted cash, beginning of period	646,832	195,088
Cash, cash equivalents and restricted cash, end of period	$ 397,876	$ 172,550