Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended				1 Months Ended	12 Months Ended
	Feb. 29, 2012	Dec. 31, 2012 Entity	Dec. 31, 2012 D Entity	Dec. 31, 2011	Sep. 30, 2012	Dec. 31, 2012 Equity Securities	Jan. 31, 2013 Common Stock Issued In Public Offering	Dec. 31, 2012 Research and Development Expense	Dec. 31, 2011 Research and Development Expense	Dec. 31, 2010 Research and Development Expense
Significant Accounting Policies [Line Items]
Number of entities that comprises the Company		4	4
Number of wholly-owned subsidiaries			3
Percentage of sharing in development cost			50.00%
Other than temporary impairment on equity securities				$ 595,000
Policy for cash equivalents			90
Policy for marketable securities			90 days
Investment in equity securities of Regulus Therapeutics Inc		38,748,000	38,748,000
Carrying value of equity securities		12,400,000	12,400,000
Unrealized gain on available securities		31,000	31,000	78,000		26,300,000
Deferred revenue		31,417,000	31,417,000	62,366,000
Deferred revenue, net of current portion		100,874,000	100,874,000	78,487,000
License fee								$ 5,700,000	$ 1,400,000	$ 2,400,000
Company ownership percentage		17.00%
Company ownership percentage					44.00%
Common stocks issued through underwritten public offering	8,625,000						9,200,000
Underwritten public offering amount per share	$ 10.75						$ 20.13