Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 82,653	$ 62,137
Short-term investments	364,255	346,596
Accounts receivable	986	1,373
Prepaid expenses and other current assets	9,938	6,617
Total current assets	457,832	416,723
Property and equipment, net	31,559	17,213
Developed technology, net	2,003	3,491
Goodwill	8,535	8,535
Other assets	4,274	1,987
Total assets	504,203	447,949
Current liabilities:
Accounts payable	2,625	2,427
Accrued expenses	21,710	11,121
Deferred revenue		232
Other current liabilities	908	140
Total current liabilities	25,243	13,920
Other long term liabilities	5,721	3,828
Total liabilities	30,964	17,748
Commitments and contingencies (Note 10)
Stockholders' equity:
Common stock, par value $0.001; 60,000 shares authorized at December 31, 2014 and 2013, respectively; 33,370 and 30,768 shares issued and outstanding at December 31, 2014 and 2013, respectively	33	31
Additional paid-in capital	920,333	684,468
Accumulated other comprehensive loss	(240)	(59)
Accumulated deficit	(446,887)	(254,239)
Total stockholders' equity	473,239	430,201
Total liabilities and stockholders' equity	$ 504,203	$ 447,949