Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities
Net loss	$ (59,601)	$ (36,424)
Adjustments:
Depreciation and amortization	1,641	1,407
Stock compensation expense	7,045	3,181
Amortization of premium on investments	1,491	687
Loss on equity method investment	254
Changes in assets and liabilities:
Accounts receivable	60	(73)
Prepaid expenses, other current assets and other assets	(868)	(2,552)
Accounts payable	(545)	(956)
Accrued expenses, other current liabilities and other liabilities	3,080	693
Net cash used in operating activities	(47,443)	(34,037)
Cash flows from investing activities
Purchase of available for sale investments	(385,244)	(167,577)
Proceeds from sales and maturities available for sale investments	136,000	90,650
Capital expenditures	(1,777)	(3,245)
Restricted cash		320
Net cash used in investing activities	(251,021)	(79,852)
Cash flows from financing activities
Proceeds from issuance of common stock, net of issuance costs	308,708	200,925
Proceeds from exercise of stock options	5,771	3,163
Payments for lease financing obligation	(79)
Net cash provided by financing activities	314,400	204,088
Effect of exchange rates on cash	(12)	(5)
Net increase in cash and equivalents	15,924	90,194
Cash and equivalents at the beginning of period	82,653	62,137
Cash and equivalents at the end of period	98,577	152,331
Capitalization of construction in progress related to facility lease obligation		1,574
Capital expenditures incurred but not yet paid	$ 213	$ 2,633