Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 53,169	$ 51,405
Marketable securities	192,701	71,407
Billed and unbilled collaboration receivables	4,248	104
Prepaid expenses and other current assets	3,910	2,641
Total current assets	254,028	125,557
Marketable securities	104,602	103,416
Investment in equity securities of Regulus Therapeutics Inc.	45,452	38,748
Property and equipment, net	16,448	19,799
Total assets	420,530	287,520
Current liabilities:
Accounts payable	5,896	4,420
Accrued expenses	14,160	11,558
Deferred rent	1,112	950
Deferred revenue	32,696	31,417
Total current liabilities	53,864	48,345
Deferred rent, net of current portion	2,925	4,248
Deferred revenue, net of current portion	93,394	100,874
Total liabilities	150,183	153,467
Commitments and contingencies (Note 6)
Stockholders' equity:
Preferred stock, $0.01 par value per share, 5,000,000 shares authorized and no shares issued and outstanding at December 31, 2013 and 2012
Common stock, $0.01 par value per share, 125,000,000 shares authorized; 63,741,573 shares issued and outstanding at December 31, 2013; 52,489,936 shares issued and outstanding at December 31, 2012	637	525
Additional paid-in capital	846,228	624,876
Accumulated other comprehensive income	19,717	15,662
Accumulated deficit	(596,235)	(507,010)
Total stockholders' equity	270,347	134,053
Total liabilities and stockholders' equity	$ 420,530	$ 287,520