CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities:
Net loss	$ (176,061)	$ (88,096)	$ (21,003)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization of property and equipment	9,917	8,023	7,593
Impairment of equipment	3,608	0	0
Share-based compensation expense	21,222	16,127	18,322
Amortization of premium on investments	(438)	167	595
Amortization of intangibles	1,153	1,153	1,529
Loss on disposal of assets	510	61	0
Changes in operating assets and liabilities:
Collaboration receivable	3,677	55,194	(49,057)
Prepaid expenses and other current assets	573	(2,098)	(1,128)
Other long-term assets	(703)	1,229	(1,692)
Accounts payable	(1,158)	7,446	(1,032)
Accrued expenses	(6,229)	(6,253)	2,043
Accrued restructuring	3,235	0	0
Collaboration liabilities	(4,537)	(23,637)	32,895
Deferred revenue	(33,438)	(5,015)	16,649
Lease incentive	5,860	4,051	0
Other current liabilities	15,182	(66)	(449)
Other long-term liabilities	2,037	2,629	3,724
Net cash (used in) provided by operating activities	(155,590)	(29,085)	8,989
Cash Flows from Investing Activities:
Purchases of property and equipment	(9,019)	(17,127)	(5,609)
Proceeds from disposal of equipment	1,447	267	0
Purchases of marketable securities	(202,525)	(524,888)	(360,008)
Proceeds from maturities of marketable securities	308,178	420,669	445,665
Net cash provided by (used in) investing activities	98,081	(121,079)	80,048
Cash Flows from Financing Activities:
Proceeds from public offering of common stock, net of issuance costs	217,786	0	0
Net proceeds from issuance of common stock under ATM facility	0	64,090	0
Proceeds from issuance of common stock under stock plans	29,272	10,258	2,407
Repurchase of common stock pursuant to share surrender	0	0	(1,066)
Net cash provided by financing activities	247,058	74,348	1,341
Net increase (decrease) in cash, cash equivalents and restricted cash	189,549	(75,816)	90,378
Cash, cash equivalents, and restricted cash, beginning of period	96,683	172,499	82,121
Cash, cash equivalents, and restricted cash, end of period	248,334	73,651
Non-Cash Activities:
Common shares issued to Parivid to settle milestone payment	0	0	3,190
Purchases of property and equipment included in accounts payable and accrued expenses	43	1,228	935
Receivable due from stock option exercises	0	93	0
Accounting Standards Update 2016-09
Non-Cash Activities:
Impact from adoption of ASUs	0	783	0
Accounting Standards Update 2014-09
Non-Cash Activities:
Impact from adoption of ASUs	$ 5,511	$ 0	$ 0