Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	$ (24,787)	$ (10,609)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,710	475
Stock-based compensation expense	5,432	2,493
Change in fair value of contingent consideration	215
Issuance of restricted common stock in exchange for consulting services to non-employees		42
Other non-cash items	130
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2)	2,064
Accounts payable	(331)	(2,388)
Accrued expenses and other liabilities	88	1,738
Deferred revenue	(6,344)	(6,335)
Deferred rent	(270)	586
Net cash used in operating activities	(24,159)	(11,934)
Investing activities
Purchase of property and equipment	(1,156)	(2,517)
Purchases of marketable securities	(223,020)
Proceeds from maturities of marketable securities	31,200
Net cash used in investing activities	(192,976)	(2,517)
Financing activities
Proceeds from issuance of common stock	2,464	671
Net cash provided by financing activities	2,464	671
Decrease in cash and cash equivalents	(214,671)	(13,780)
Cash and cash equivalents at beginning of period	347,845	206,279
Cash and cash equivalents at end of period	133,174	192,499
Non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses	$ 1,125	$ 947