Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities
Net loss	$ (49,762)	$ (40,375)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	11,955	14,528
Amortization of premiums and accretion of discounts on investment securities, available for sale	(234)	(289)
Depreciation	138	122
Loss on disposal of assets	5
Changes in operating assets and liabilities:
Interest and other receivables	855	218
Prepaid expenses and other current assets	(2,107)	(92)
Other assets	(911)	9
Accounts payable	(365)	408
Accrued liabilities	(771)	42
Long-term liabilities	(15)	83
Net cash used in operating activities	(41,212)	(25,346)
Cash flows from investing activities
Purchases of investment securities	(240,566)	(49,646)
Maturities of investment securities	72,077	89,107
Purchases of property and equipment	(207)	(988)
Net cash (used in) provided by investing activities	(168,696)	38,473
Cash flows from financing activities
Proceeds from issuance of common stock, net of issuance costs	282,991	1,439
Net cash provided by financing activities	282,991	1,439
Effect of exchange rate changes on cash	(1)	4
Net increase in cash and cash equivalents	73,082	14,570
Cash and cash equivalents
Beginning of period	102,138	61,854
End of period	175,220	76,424
Supplemental disclosure of noncash investing information:
Property and equipment purchases in accounts payable and accrued liabilities	$ 356	$ 367