Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Operating activities
Net loss	$ (35,441)	$ (24,105)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,860	5,053
Amortization of investment premiums and discounts	2,294	880
Depreciation expense	79	11
Loss on foreign exchange	4
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(816)	(3,314)
Other assets	7	(24)
Accounts payable	1,790	1,162
Accrued compensation	(489)	(301)
Accrued research and development expenses	(1,313)	4,500
Other accrued liabilities	235	3,247
Long-term liabilities	511	27
Net cash used in operating activities	(23,279)	(12,864)
Investing activities
Purchases of short-term investments	(186,006)	(111,325)
Sales of short-term investments	95,242	29,589
Maturities of short-term investments	118,320	33,920
Purchases of property and equipment	(1,639)	(5)
Net cash provided by (used in) investing activities	25,917	(47,821)
Financing activities
Proceeds from sale of common stock, net of offering costs		69,445
Taxes paid related to net share settlement of restricted stock units	(53)	(4,468)
Proceeds from exercise of stock options	29
Net cash provided by (used in) financing activities	(24)	64,977
Effect of exchange rates on cash	(4)
Increase in cash and cash equivalents	2,610	4,292
Cash and cash equivalents at beginning of period	23,746	21,897
Cash and cash equivalents at end of period	26,356	26,189
Non-cash investing and financing activities
Issuance of common stock upon vesting of stock awards	40	40
Change in long-term liabilities related to non-vested stock awards	(40)	(40)
Proceeds receivable from option exercises		32
Offering costs in anticipation of public filing included in other accrued liabilities and accounts payable		312
Property and equipment purchases included in liabilities	503
Supplemental cash flow disclosure
Cash paid for taxes	$ 3	$ 2