Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Operating activities
Net loss	$ (29,165)	$ (17,240)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Noncash benefit on release of tax valuation allowance	(11,797)
Depreciation and amortization	2,558	610
Stock-based compensation expense	7,757	4,833
Remeasurement of contingent consideration	78
Issuance of restricted common stock in exchange for consulting services to non-employees	168
Re-measurement of warrants		440
Loss on disposal of equipment	1	2
Other non-cash items	77
Changes in operating assets and liabilities:
Prepaid expenses and other assets	166	(3,903)
Accounts payable	(2,604)	(1,021)
Accrued expenses and other liabilities	5,613	1,248
Deferred revenue	(19,005)	61,203
Deferred rent	2,124	2,733
Net cash (used in) provided by operating activities	(44,029)	48,905
Investing activities
Restricted cash	209	(1,153)
Purchase of property and equipment	(6,303)	(3,380)
Purchases of marketable securities	(174,021)
Acquisition of business, net of cash acquired	(4,673)
Net cash used in investing activities	(184,788)	(4,533)
Financing activities
Proceeds from public offering of common stock, net of issuance costs	109,766	104,972
Repayment of nonrecourse note collateralized by restricted stock		344
Proceeds from exercise of stock options and issuance of common stock	2,375	92
Net cash provided by financing activities	112,141	105,408
(Decrease) increase in cash and cash equivalents	(116,676)	149,780
Cash and cash equivalents at beginning of period	206,279	67,011
Cash and cash equivalents at end of period	89,603	216,791
Non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses	1,298	1,347
Reclassification of warrants to additional paid-in capital		655
Conversion of preferred stock to common stock upon closing of IPO		122,177
Stock option exercise proceeds receivable	$ 223