CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 73,651	$ 150,738
Marketable securities	269,017	202,413
Collaboration receivable	15,048	70,242
Prepaid expenses and other current assets	6,798	4,607
Restricted cash	2,412	0
Total current assets	366,926	428,000
Marketable securities, long-term	37,222	0
Property and equipment, net	29,916	20,847
Restricted cash	20,620	21,761
Intangible assets, net	4,036	5,189
Other long-term assets	711	1,940
Total assets	459,431	477,737
Current liabilities:
Accounts payable	11,456	3,632
Accrued expenses	20,528	26,866
Collaboration liabilities	9,258	32,895
Deferred revenue	2,866	7,272
Other current liabilities	379	11
Total current liabilities	44,487	70,676
Deferred revenue, net of current portion	30,751	31,360
Other long-term liabilities	10,039	3,793
Total liabilities	85,277	105,829
Commitments and contingencies (Note 14)
Stockholders' Equity:
Common stock, $0.0001 par value per share; 100,000 shares authorized, 76,584 shares issued and 76,355 shares outstanding at December 31, 2017 and 71,305 shares issued and 71,076 shares outstanding at December 31, 2016	8	7
Additional paid-in capital	939,654	848,304
Accumulated other comprehensive (loss) income	(140)	86
Accumulated deficit	(562,254)	(473,375)
Treasury stock, at cost, 229 shares	(3,114)	(3,114)
Total stockholders' equity	374,154	371,908
Total liabilities and stockholders' equity	$ 459,431	$ 477,737