CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities:
Net loss	$ (50,777)	$ (250,943)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,690	2,600
Non-cash stock-based compensation	8,236	5,591
Charge for 401(k) company stock match	247	157
Benefit from intraperiod tax allocation	(431)	(17,870)
In-process research and development		224,656
Changes in operating assets and liabilities:
Billed and unbilled collaboration receivables	29,830	4,084
Prepaid expenses and other assets	(3,179)	(4,499)
Accounts payable	750	(1,379)
Accrued expenses and other	(8,432)	3,773
Deferred revenue	(6,189)	(7,845)
Net cash used in operating activities	(26,255)	(41,675)
Cash flows from investing activities:
Purchases of property and equipment	(2,110)	(388)
Increase in restricted cash	(288)
Purchases of marketable securities	(507,617)	(487,598)
Sales and maturities of marketable securities	168,687	133,206
Payment for asset acquisition		(25,000)
Net cash used in investing activities	(341,328)	(379,780)
Cash flows from financing activities:
Proceeds from exercise of stock options and other types of equity	13,566	11,541
Proceeds from issuance of common stock, net of offering costs	496,400
Proceeds from issuance of common stock to Genzyme	89,018	723,037
Payments for repurchase of common stock for employee tax withholding		(15,703)
Net cash provided by financing activities	598,984	718,875
Net increase in cash and cash equivalents	231,401	297,420
Cash and cash equivalents, beginning of period	75,179	53,169
Cash and cash equivalents, end of period	$ 306,580	$ 350,589