Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenues
Net product sales	$ 2,711	$ 3,518
Toll manufacturing	718	1,023
Total revenues	6,624	9,847
Operating Costs and Expenses
Cost of product sales	2,532	2,428
Cost of toll manufacturing	919	1,188
Research and development	15,511	18,502
General and administrative	8,164	6,924
Total operating costs and expenses	27,126	29,042
Loss from operations	(20,502)	(19,195)
Interest and Other Income (Expense)
Interest income	34	88
Interest expense	(1,570)	(1,679)
Other	(459)	(762)
Total interest and other expense, net	(1,995)	(2,353)
Net loss	(22,497)	(21,548)
Less net loss attributable to noncontrolling interest in consolidated variable interest entity	444
Net loss attributable to stockholders of Arena	$ (22,053)	$ (21,548)
Net loss attributable to stockholders of Arena per share:
Basic	$ (0.09)	$ (0.09)
Diluted	$ (0.09)	$ (0.09)
Shares used in calculating net loss attributable to stockholders of Arena per share:
Basic	244,822	242,876
Diluted	244,822	242,876
Comprehensive Loss:
Net loss	$ (22,497)	$ (21,548)
Foreign currency translation gain (loss)	804	2,591
Comprehensive loss	(21,693)	(18,957)
Less comprehensive loss attributable to noncontrolling interest in consolidated variable interest entity	444
Comprehensive loss attributable to stockholders of Arena	(21,249)	(18,957)
Eisai
Revenues
Other collaboration revenue	1,535	3,226
Other
Revenues
Other collaboration revenue	$ 1,660	$ 2,080