Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities
Net loss	$ (82,177)	$ (57,513)	$ (18,497)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	194	107	17
Loss on disposition of assets		26
Stock-based compensation expense	10,699	4,641	215
Amortization of debt discount			121
Non-cash interest expense			7
Remeasurement of convertible preferred stock call option liability		9,560	(969)
Remeasurement of convertible preferred stock warrant liability		2,279	41
Changes in assets and liabilities
Prepaid expenses and other assets	317	(1,695)	383
Accounts payable	413	944	(675)
Accrued liabilities and other liabilities	1,490	1,998	2,267
Net cash (used in) operating activities	(69,064)	(39,653)	(17,090)
Cash flows from investing activities
Purchase of property and equipment		(827)	(9)
Security deposit for facility lease	(42)	55
Net cash used in investing activities	(42)	(772)	(9)
Cash flows from financing activities
Proceeds from issuance of common stock in initial public offering, net of issuance costs		132,137
Proceeds from issuance of convertible preferred stock, net of issuance costs		64,793	29,956
Proceeds from exercise of convertible preferred stock warrants		572
Proceeds from issuance of common stock in connection with employee benefit plans	400	201	27
Net cash provided by financing activities	80,609	197,703	29,983
Net increase in cash and cash equivalents	11,503	157,278	12,884
Cash and cash equivalents at beginning of period	170,566	13,288	404
Cash and cash equivalents at end of period	182,069	170,566	13,288
Supplemental disclosure of noncash items
Conversion of notes payable and accrued interest to preferred stock			4,588
Issuance of call options related to convertible preferred stock			$ 1,116
Conversion of preferred stock call option liability [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital		9,581
Conversion of preferred stock warrant liability [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital		2,752
Conversion of preferred stock to common stock [Member]
Supplemental disclosure of noncash items
Conversion to common stock and additional paid in capital		$ 122,290
Secondary Offering [Member]
Cash flows from financing activities
Proceeds from issuance of common stock in public offering, net of issuance costs	$ 80,209