Significant Accounting Policies (Details 3) (USD $)	1 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2013	Sep. 30, 2013 entity	Dec. 31, 2012 entity
Public Offering
Shares sold	9,617,869
Issuance price per share (in dollars per share)	$ 19.00
Additional shares sold pursuant to option granted to the underwriters	617,869
Proceeds from issuance of common stock	$ 173,200,000	$ 173,292,000
Underwriting discounts and commissions and other estimated offering expenses	9,500,000
Consolidation of variable interest entities
Number of variable interest entities in which the entity is not primary beneficiary		5	6
Carrying value of investments in variable interest entities		$ 66,800,000
Maximum
Equity method of accounting
Ownership percentage		20.00%
Regulus Therapeutics Inc. | Maximum
Equity method of accounting
Ownership percentage		20.00%