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	$ (25,787)	$ (22,704)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation	291	223
Amortization of deferred financing costs and debt discount	76	75
Amortization and accretion of premium or discount on available-for-sale investments, net	52	147
Stock-based compensation	4,850	3,534
Changes in operating assets and liabilities
Prepaid, current and other assets	1,427	503
Accounts payable and other current liabilities	(5,733)	(1,130)
Interest payable	(30)	(6)
Net cash used in operating activities	(24,854)	(19,358)
Cash flows from investing activities
Purchase of available-for-sale investments	(45,561)	(13,265)
Maturity of available-for-sale investments	12,860	16,036
Purchase of property, plant and equipment	(904)	(335)
Net cash (used in) provided by investing activities	(33,605)	2,436
Cash flows from financing activities
Proceeds from exercise of stock options	551	4
Proceeds from exercise of stock purchase rights	148	252
Tax withholdings related to restricted stock awards	(651)	(128)
Net cash provided by financing activities	48	128
Net change in cash and cash equivalents	(58,411)	(16,794)
Beginning of period	197,945	91,060
End of period	139,534	74,266
Supplemental disclosures
Income taxes paid	0	1,789
Interest paid	551	551
Amounts capitalized under build-to-suit lease transaction	4,204	0
Interest capitalized during construction period for build-to-suit lease transaction	$ 46	$ 0