Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Cash flows from operating activities
Net loss	$ (36,424)	$ (14,484)
Adjustments:
Depreciation and amortization	1,407	1,314
Stock compensation expense	3,181	1,716
Amortization of premium on investments	687	674
Changes in assets and liabilities:
Accounts receivable	(73)	6
Prepaid expenses, other current assets and other assets	(2,552)	(2,482)
Accounts payable	(956)	(495)
Accrued expenses, other current liabilities and other liabilities	832	(481)
Deferred revenue	(139)	(2,919)
Net cash used in operating activities	(34,037)	(17,151)
Cash flows from investing activities
Purchase of available for sale investments	(167,577)	(103,776)
Proceeds from sales and maturities available for sale investments	90,650	43,000
Capital expenditures	(3,245)	(2,412)
Restricted cash	320	(2,868)
Net cash used in investing activities	(79,852)	(66,056)
Cash flows from financing activities
Proceeds from issuance of common stock, net of issuance costs	200,925	111,055
Proceeds from exercise of stock options	3,163	2,034
Net cash provided by financing activities	204,088	113,089
Effect of exchange rate on cash	(5)	(3)
Net increase in cash and equivalents	90,194	29,879
Cash and equivalents at the beginning of period	62,137	23,883
Cash and equivalents at the end of period	152,331	53,762
Capitalization of construction in progress related to facility lease obligation (Note 8)	1,574
Capital expenditures incurred but not yet paid	$ 2,633