CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (83,118)	$ (64,544)	$ (43,698)
Non-cash adjustments reconciling net loss to operating cash flows
Depreciation of property and equipment	180	212	199
Amortization of premium on investments	7	337	534
Share-based compensation expense	10,624	10,254	7,050
Change in fair value of warrant liability	0	0	4,517
Changes in operating assets and liabilities
Other current assets	254	279	(1,209)
Other long-term assets	(1,259)	(1,675)	(326)
Accounts payable and accrued liabilities	5,196	4,370	151
Other current and long term liabilities	99	53	21
Cash flows used in operating activities	(68,017)	(50,714)	(32,748)
Investing activities:
Purchases of short-term investments	(70,269)	(104,954)	(10,468)
Disposal and maturities of short-term investments	108,720	54,530	35,073
Purchases of property and equipment	(196)	(329)	(386)
Cash flows provided by (used in) investing activities	38,255	(50,753)	24,219
Financing activities:
Proceeds from issuance of common stock, net of issuance costs	0	143,293	0
Proceeds from issuance under employee stock purchase plan	297	522	0
Proceeds from exercise of common stock options and warrants	2,355	552	887
Cash flows provided by financing activities	2,652	144,367	887
Increase (decrease) in cash and cash equivalents	(27,110)	42,900	(7,642)
Cash and cash equivalents, beginning of year	49,493	6,593	14,235
Cash and cash equivalents, end of year	22,383	49,493	6,593
Net exercise of warrants	$ 0	$ 0	$ 993