Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash flows from operating activities
Net loss	$ (130,685)	$ (91,754)	$ (79,676)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities
Non-cash interest expense	4,548	78
Depreciation and amortization	2,589	3,664	4,596
Stock-based compensation	10,733	6,889	6,952
Other non-cash items	579	(587)	864
Changes in operating assets and liabilities:
Purchased premiums and interest on available-for-sale securities	(1,809)	(2,354)
Accounts receivable	3,410	(1,841)	(3,681)
Prepaid expenses and other current assets	4,037	(2,477)	(3,933)
Accounts payable, accrued expenses and other	13,304	6,985	8,815
Deferred revenues	(4,989)	(5,281)	11,963
Deferred rent and tax incentives	2,076	7,892	1,264
Other assets and liabilities, net	1,032	(1,030)	19
Net cash used in operating activities	(95,175)	(79,816)	(52,817)
Cash flows from investing activities
Purchase of available-for-sale securities	(112,923)	(115,665)
Proceeds from sales and maturities of available-for-sale securities	95,100	43,880
Purchase of property and equipment	(9,857)	(3,189)	(3,754)
Assignment of restricted cash	(57)	(628)
Release of restricted cash		381
Other investing activities, net	(2)		7
Net cash used in investing activities	(27,739)	(75,221)	(3,747)
Cash flows from financing activities
Proceeds from public offerings, net of offering costs	26,716	100,025
Proceeds from issuance of convertible preferred stock, net of offering costs			76,949
Proceeds from exercise of common stock and warrants	2,036	5,427	1,745
Principal payment on capital lease obligations		(48)	(443)
Proceeds from issuance of debt, net of issuance costs	121,537	41,128
Payments of dividends on Series B convertible preferred stock	(3)	(4,235)
Deferred financing costs			(1,946)
Net cash provided by financing activities	150,286	142,297	76,305
Net increase (decrease) in cash and cash equivalents	27,372	(12,740)	19,741
Cash and cash equivalents, beginning of period	37,714	50,454	30,713
Cash and cash equivalents, end of period	65,086	37,714	50,454
Noncash financing and investing activities
Conversion of convertible preferred stock to common stock		268,225
Conversion of convertible preferred stock warrants to common stock warrants		929
Issuance of derivative liability	35	196
Value of conversion feature of convertible senior notes, classified in Stockholders' Deficit	51,876
Changes in property and equipment in accounts payable and accrued expenses		412
Disposals of fully depreciated assets	210	671	0
Reclassification of deferred financing costs to stockholders' deficit	278	2,748
Dividends on Series B convertible preferred stock declared but not paid		28
Conversion of Silver Creek convertible notes	2,603
Supplemental disclosure of cash flows
Cash paid for interest	$ 3,915	$ 169	$ 13