Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Cash flows from operating activities
Net loss	$ (43,541)	$ (42,731)
Adjustments to reconcile net loss to net cash used in operating activities
Remeasurement of convertible preferred stock warrants	(587)	576
Amortization of premium on available-for-sale securities	341
Amortization of deferred lease benefits and tax incentives	(295)	(385)
Depreciation and amortization	2,217	2,670
Stock-based compensation	2,749	3,784
Changes in operating assets and liabilities
Purchased premiums and interest on available-for-sale securities	(1,760)
Accounts receivable	486	(1,027)
Prepaid expenses and other current assets	(281)	(1,184)
Accounts payable	(1,911)	1,355
Accrued expenses and other	1,258	1,455
Deferred revenues	553	3,916
Deferred lease benefits and tax incentives	329	1,212
Other assets and liabilities, net		14
Net cash used in operating activities	(40,442)	(30,345)
Cash flows from investing activities
Purchase of available-for-sale securities	(73,825)
Purchase of property and equipment	(891)	(1,833)
Other investing activities, net	(100)	1
Net cash used in investing activities	(74,816)	(1,832)
Cash flows from financing activities
Proceeds from initial public offering, net of offering costs	100,025
Proceeds from issuance of convertible preferred stock, net of offering costs		76,949
Proceeds from exercise of stock options	847	457
Proceeds from exercise of stock warrants	26
Principal payments on capital lease obligations	(48)	(307)
Payment of dividends on Series B convertible preferred stock	(4,003)
Net cash provided by financing activities	96,847	77,099
Net increase (decrease) in cash and cash equivalents	(18,411)	44,922
Cash and cash equivalents, beginning of period	50,454	30,713
Cash and cash equivalents, end of period	32,043	75,635
Non-cash investing and financing activities
Conversion of convertible preferred stock to common stock	268,225
Conversion of convertible preferred stock warrants to common stock warrants	929
Reclassification of deferred financing costs to stockholders' equity	2,748
Dividends on Series B convertible preferred stock declared but not paid	261
Supplemental disclosure of cash flows
Cash paid for interest		$ 10