Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities
Net loss	$ (29,722)	$ (24,231)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	54	53
Stock-based compensation expense	3,864	2,363
Changes in assets and liabilities
Prepaid expenses and other assets	(9,065)	(255)
Accounts payable	220	(293)
Accrued and other liabilities	(79)	1,314
Net cash used in operating activities	(34,728)	(21,049)
Cash flows from investing activities
Purchase of property and equipment		(90)
Change in restricted cash	(2,378)
Net cash used in investing activities	(2,378)	(90)
Cash flows from financing activities
Proceeds from issuance of common stock in connection with employee benefit plans	1,034	3
Net cash provided by financing activities	1,034	3
Net increase (decrease) in cash and cash equivalents	(36,072)	(21,136)
Cash and cash equivalents at beginning of period	201,153	182,069
Cash and cash equivalents at end of period	165,081	$ 160,933
Supplemental disclosure of noncash items
Build-to-suit lease transaction	$ 8,174