Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities:
Net loss	$ (135,747)	$ (60,716)	$ (37,034)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	296	96	49
Amortization of premium/discount on investments	494	558	268
Loss on disposal of property and equipment	306
Stock-based compensation	5,825	4,714	6,010
Changes in operating assets and liabilities:
Unbilled receivable	(33,823)
Prepaid expenses and other current assets	428	(1,001)	(686)
Other long-term assets	(2)	(976)	(179)
Accounts payable	(274)	231	1,307
Accrued expense	20,703	4,646	2,754
Deferred revenue	197,289
Deferred rent	2,411	41	28
Net cash provided by (used in) operating activities	57,906	(52,407)	(27,483)
Investing activities:
Purchase of equipment	(2,662)	(414)	(229)
Proceeds from the maturities of available for sale securities	162,376	63,901	12,585
Purchase of available for sale securities	(147,009)	(77,175)	(77,708)
Net cash provided by (used in) investing activities	12,705	(13,688)	(65,352)
Financing activities:
Proceeds from the issuance of common stock, net of issuance costs	66,736	82,750	104,328
Proceeds from the sale of stock under employee stock purchase plan	106	220
Proceeds from the exercise of stock options	124	130
Repurchase of treasury stock			(162)
Payments received on promissory notes issued in exchange for shares of common stock			237
Payments on capital lease obligations	(20)	(7)	(3)
Net cash provided by financing activities	66,946	83,093	104,400
Increase in cash and cash equivalents	137,557	16,998	11,565
Cash and cash equivalents at beginning of the period	49,778	32,780	21,215
Cash and cash equivalents at end of the period	187,335	49,778	32,780
Non-cash financing activities
Conversion of series A, series B and series C preferred stock into common stock			244,727
Accretion of preferred stock to redemption value			$ 86,899
Unpaid follow-on offering costs	$ 12	102
Assets acquired under capital lease		$ 12