Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities:
Net loss	$ (19,704)	$ (7,971)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	3,904	1,327
Depreciation and amortization expense	386	92
Gain from revaluation of preferred stock warrant liability		(213)
Non-cash interest expense	54	30
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,040)	(710)
Deferred revenue	117,290
Accounts payable	1,331	(189)
Accrued expenses and other current liabilities	(806)	(706)
Net cash provided by (used in) operating activities	101,415	(8,340)
Cash flows from investing activities:
Purchases of property and equipment	(3,400)	(214)
Purchases of investments	(45,301)	(59,255)
Maturities of investments	66,425
Changes in restricted cash	(1)
Net cash provided by (used in) investing activities	17,723	(59,469)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		(24)
Proceeds from exercise of stock options and common stock warrants	157	89
Repayment of notes payable		(300)
Payments of initial public offering costs		(1,096)
Net cash provided by (used in) financing activities	157	(1,331)
Net increase (decrease) in cash and cash equivalents	119,295	(69,140)
Cash and cash equivalents at beginning of period	73,933	114,185
Cash and cash equivalents at end of period	193,228	45,045
Supplemental disclosure of cash flow information:
Cash paid for interest	10	40
Supplemental disclosure of non-cash investing and financing activities:
Deferred offering costs included in accounts payable and accrued expenses		162
Property and equipment purchases included in accounts payable and accrued expenses	$ 3,395	$ 28