Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 350,589	$ 53,169
Marketable securities	314,901	192,701
Billed and unbilled collaboration receivables	164	4,248
Prepaid expenses and other current assets	8,409	3,910
Total current assets	674,063	254,028
Marketable securities	335,025	104,602
Investment in equity securities of Regulus Therapeutics Inc.	55,478	45,452
Property and equipment, net	15,731	16,448
Total assets	1,080,297	420,530
Current liabilities:
Accounts payable	5,037	5,896
Accrued expenses	16,600	14,160
Liability for stock issuance	25,379
Accrued intraperiod tax allocation	6,825
Deferred rent	869	1,112
Deferred revenue	29,121	32,696
Total current liabilities	83,831	53,864
Deferred rent, net of current portion	2,819	2,925
Deferred revenue, net of current portion	37,674	93,394
Other liabilities	1,000
Total liabilities	125,324	150,183
Commitments and contingencies (Note 3)
Stockholders' equity:
Preferred stock, $0.01 par value, 5,000,000 shares authorized and no shares issued and outstanding at March 31, 2014 and December 31, 2013
Common stock, $0.01 par value, 125,000,000 shares authorized; 75,484,782 shares issued and outstanding at March 31, 2014; 63,741,573 shares issued and outstanding at December 31, 2013	755	637
Additional paid-in capital	1,776,366	846,228
Accumulated other comprehensive income	25,030	19,717
Accumulated deficit	(847,178)	(596,235)
Total stockholders' equity	954,973	270,347
Total liabilities and stockholders' equity	$ 1,080,297	$ 420,530