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	$ (74,363)	$ (48,133)	$ (31,148)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation	252	73	64
Amortization and accretion costs related to notes payable—related parties	0	0	3,207
Amortization of deferred financing costs and debt discount	305	77	0
Amortization and accretion of premium or discount on available-for-sale investments, net	570	416	0
Interest payable—related parties	0	0	588
Loss (gain) on conversion of notes payable	0	0	2,737
Stock-based compensation	12,945	9,178	2,858
Change in fair value measurements	0	0	3,717
Changes in operating assets and liabilities
Prepaid, current and other assets	(840)	(717)	(516)
Accounts payable and other current liabilities	5,385	4,829	2,045
Interest payable	0	551	0
Net cash used in operating activities	(55,746)	(33,726)	(16,448)
Cash flows from investing activities
Purchase of available-for-sale investments	(46,872)	(95,376)	0
Maturity of available-for-sale investments	55,785	20,739	0
Sale of available-for-sale investments	3,749	1,500	0
Purchase of furniture, fixtures and equipment	(3,280)	(181)	(63)
Net cash provided by (used in) investing activities	9,382	(73,318)	(63)
Cash flows from financing activities
Proceeds from sale of common stock, net of commissions	50,451	0	0
Proceeds from exercise of stock options	1,282	9	1
Proceeds from exercise of stock purchase rights	96	119	0
Proceeds from exercise of warrants	9	0	8
Payments of debt issuance costs	0	122,853	0
Payments of initial public offering costs	0	0	71,870
Proceeds from notes payable to related parties	0	0	(3,644)
Proceeds from notes payable to related parties	0	0	15,000
Net cash provided by financing activities	51,838	122,981	83,235
Net change in cash and cash equivalents	5,474	15,937	66,724
Cash and cash equivalents
Beginning of period	85,586	69,649	2,925
End of period	91,060	85,586	69,649
Supplemental disclosures
Interest paid	2,186	0	0
Income taxes paid	600	0	0
Non-cash financing activities
Conversion of preferred stock to common stock	0	0	61,348
Conversion of convertible notes payable and accrued interest to common stock	0	0	18,604
Issuance of common stock upon net exercise of warrants	0	0	4,888
Reclassification of warrants from liabilities to equity	0	0	6,560
Accretion from conversion of note payable to related parties	0	0	240
Accretion of stock issuance costs	0	0	210
Deferred costs from issuance of convertible notes	$ 0	$ 25	$ 0