Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities:
Net loss	$ (72,865)	$ (70,972)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	277	208
Stock-based compensation	29,764	29,510
Changes in operating assets and liabilities:
Prepaid expenses and other	516	(170)
Accounts payable	3,935	7,034
Accrued expenses	2,334	(571)
Accrual of deferred rent	(3)	(67)
Net cash used in operating activities	(36,042)	(35,028)
Investing activities:
Purchase of property and equipment	(124)	(44)
Restricted cash	1	(1)
Purchase of available-for-sale securities	(79,728)	(36,768)
Sale/maturity of available-for-sale securities	25,779	118,269
Net cash (used in) provided by investing activities	(54,072)	81,456
Financing activities:
Net proceeds from exercise of options	706	222
Net cash provided by financing activities	706	222
Net increase in cash and cash equivalents	(89,408)	46,650
Cash and cash equivalents, beginning of period	194,494	31,569
Cash and cash equivalents, end of period	$ 105,086	$ 78,219