CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash Flows from Operating Activities:
Net loss	$ (83,313)	$ (98,600)	$ (108,408)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization of property and equipment	7,594	7,637	7,108
Share-based compensation expense	11,440	13,594	12,822
Amortization of premium on investments	1,383	2,162	3,575
Amortization of intangibles	1,061	1,061	1,061
Impairment of equity investment			244
Loss on disposal of assets			23
Changes in operating assets and liabilities:
Accounts receivable	(11,958)	5,668	(2,284)
Unbilled revenue	1,109	504	(2,613)
Prepaid expenses and other current assets	(14)	(64)	1,552
Restricted cash	59		(748)
Other long-term assets	(92)
Accounts payable	(3,380)	1,126	2,727
Accrued expenses	14,151	(1,099)	1,806
Deferred revenue	(9,015)	3,282	(3,979)
Other current liabilities	(58)	22	(18)
Other long-term liabilities	(482)	(461)	300
Net cash used in operating activities	(71,515)	(65,168)	(86,832)
Cash Flows from Investing Activities:
Purchases of property and equipment	(4,068)	(8,360)	(9,450)
Purchases of marketable securities	(405,673)	(111,809)	(229,969)
Proceeds from maturities of marketable securities	245,907	195,342	294,183
Proceeds from sales of marketable securities			3,822
Net cash (used in) provided by investing activities	(163,834)	75,173	58,586
Cash Flows from Financing activities:
Proceeds from public offering of common stock, net of issuance costs	148,439
Net proceeds from issuance of common stock under ATM facility	64,503	18,305
Proceeds from issuance of common stock under stock plans	24,567	3,273	5,022
Repurchase of common stock pursuant to share surrender	(2,048)
Net cash provided by financing activities	235,461	21,578	5,022
Increase (decrease) in cash and cash equivalents	112	31,583	(23,224)
Cash and cash equivalents, beginning of period	61,349	29,766	52,990
Cash and cash equivalents, end of period	$ 61,461	$ 61,349	$ 29,766