Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating activities:
Net loss	$ (70,972)	$ (52,454)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	208	180
Build-out allowance received from landlord		179
Stock-based compensation	29,510	20,103
Changes in operating assets and liabilities:
Prepaid expenses and other	(170)	131
Accounts payable	7,034	(2,389)
Accrued expenses	(571)	(15,975)
Accrual of deferred rent	(67)	202
Net cash used in operating activities	(35,028)	(50,023)
Investing activities:
Purchase of property and equipment	(44)	(435)
Restricted cash	(1)
Expenditures for leasehold improvements		(179)
Purchase of available-for-sale securities	(36,768)	(104,838)
Sale/maturity of available-for-sale securities	118,269	52,769
Net cash provided by (used in) investing activities	81,456	(52,683)
Financing activities:
Net proceeds from issuance of common stock		205,196
Net proceeds from exercise of options	222	14,437
Net cash provided by financing activities	222	219,633
Net increase in cash and cash equivalents	46,650	116,927
Cash and cash equivalents, beginning of period	31,569	38,539
Cash and cash equivalents, end of period	$ 78,219	$ 155,466