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	$ (164,443)	$ (92,475)	$ (37,948)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	40,194	16,039	5,711
Amortization of premiums and accretion of discounts on investment securities, available for sale	(2,060)	484	1,528
Depreciation	647	206	79
Income tax benefit from exercise of stock options	(247)
Gain on disposal of assets			(10)
Changes in operating assets and liabilities:
Interest and other receivables	(674)	(214)	(505)
Prepaid expenses and other current assets	(804)	652	(1,484)
Restricted cash	(375)
Other assets	(42)	(108)	(179)
Accounts payable	(344)	1,644	(1,003)
Accrued liabilities	6,256	7,266	2,413
Deferred revenue		(55)	(379)
Long-term liabilities	97	127	8
Net cash used in operating activities	(121,795)	(66,434)	(31,769)
Cash flows from investing activities
Purchases of investment securities	(269,486)	(335,361)	(211,585)
Maturities of investment securities	419,197	248,268	86,087
Purchases of property and equipment	(2,141)	(180)	(618)
Proceeds from sales of property and equipment			12
Net cash provided by (used in) investing activities	147,570	(87,273)	(126,104)
Cash flows from financing activities
Proceeds from issuances of equity securities, net of issuance costs	14,547	203,851	111,682
Deferred offering costs	(292)
Income tax benefit from exercise of stock options	247
Net cash provided by financing activities	14,502	203,851	111,682
Effect of exchange rate changes on cash	7	3	(1)
Net increase (decrease) in cash and cash equivalents	40,284	50,147	(46,192)
Cash and cash equivalents at beginning of period	61,854	11,707	57,899
Cash and cash equivalents at end of period	102,138	$ 61,854	$ 11,707
Supplemental disclosure of cash flow information:
Cash paid for income taxes	415
Supplemental disclosure of noncash investing information:
Property and equipment purchases in accrued liabilities	$ 156