Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Cash flows from operating activities:
Net loss	$ (250,943)	$ (9,013)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,600	2,127
Non-cash stock-based compensation	5,591	3,075
Charge for 401(k) company stock match	157	120
Benefit from intraperiod tax allocation	(17,870)	(562)
In-process research and development	224,656
Changes in operating assets and liabilities:
Proceeds from landlord tenant improvements		192
Billed and unbilled collaboration receivables	4,084	(454)
Prepaid expenses and other assets	(4,499)	(944)
Accounts payable	(1,379)	(959)
Accrued expenses and other	3,773	(1,790)
Deferred revenue	(7,845)	7,237
Net cash used in operating activities	(41,675)	(971)
Cash flows from investing activities:
Purchases of property and equipment	(388)	(1,799)
Purchases of marketable securities	(487,598)	(187,077)
Sales and maturities of marketable securities	133,206	58,036
Payment for asset acquisition	(25,000)
Net cash used in investing activities	(379,780)	(130,840)
Cash flows from financing activities:
Proceeds from exercise of stock options and other types of equity	11,541	4,638
Proceeds from issuance of common stock, net of offering costs		173,572
Proceeds from issuance of common stock to Genzyme	723,037
Payments for repurchase of common stock for employee tax withholding	(15,703)
Net cash provided by financing activities	718,875	178,210
Net increase in cash and cash equivalents	297,420	46,399
Cash and cash equivalents, beginning of period	53,169	51,405
Cash and cash equivalents, end of period	$ 350,589	$ 97,804