CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 180,895	$ 75,179
Marketable securities	848,217	526,929
Investment in equity securities of Regulus Therapeutics Inc.	51,419	94,583
Billed and unbilled collaboration receivables	8,298	39,937
Prepaid expenses and other current assets	16,559	9,739
Total current assets	1,105,388	746,367
Marketable securities	251,839	279,821
Deferred tax assets		31,667
Property and equipment, net	27,812	21,740
Other assets	1,471
Total assets	1,386,510	1,079,595
Current liabilities:
Accounts payable	16,787	15,111
Accrued expenses	28,798	23,680
Deferred tax liabilities		31,667
Deferred rent	1,162	1,005
Deferred revenue	15,352	23,871
Total current liabilities	62,099	95,334
Deferred rent, net of current portion	5,431	5,011
Deferred revenue, net of current portion	52,965	42,983
Other liabilities	1,301
Total liabilities	$ 121,796	$ 143,328
Commitments and contingencies (Note 7)
Stockholders' equity:
Preferred stock, $0.01 par value per share, 5,000,000 shares authorized and no shares issued and outstanding at December 31, 2015 and 2014
Common stock, $0.01 par value per share, 125,000,000 shares authorized; 85,090,968 shares issued and outstanding at December 31, 2015; 77,202,753 shares issued and outstanding at December 31, 2014	$ 851	$ 772
Additional paid-in capital	2,506,197	1,843,362
Accumulated other comprehensive income	4,369	48,763
Accumulated deficit	(1,246,703)	(956,630)
Total stockholders' equity	1,264,714	936,267
Total liabilities and stockholders' equity	$ 1,386,510	$ 1,079,595