Strategic Alliances and Collaboration - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended		12 Months Ended		1 Months Ended	12 Months Ended									1 Months Ended	12 Months Ended
	Aug. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Sanofi Three Remaining Micro RNA Targets	Dec. 31, 2012 Sanofi Each of Four Micro RNA Targets	Oct. 31, 2012 Astra Zeneca	Dec. 31, 2012 Astra Zeneca Agreement	Dec. 31, 2012 Astra Zeneca Preclinical	Dec. 31, 2012 Astra Zeneca Preclinical Three Remaining Micro RNA Targets	Dec. 31, 2012 Astra Zeneca Preclinical Each of Four Micro RNA Targets	Dec. 31, 2012 Astra Zeneca Clinical	Dec. 31, 2012 Astra Zeneca Commercialization	Dec. 31, 2012 Astra Zeneca Minimum	Dec. 31, 2012 Astra Zeneca Maximum	Dec. 31, 2012 Immuno-Inflammatory Alliance	Jul. 30, 2011 Immuno-Inflammatory Alliance Preclinical	Dec. 31, 2012 Immuno-Inflammatory Alliance Preclinical	Dec. 31, 2012 Immuno-Inflammatory Alliance Clinical	Dec. 31, 2012 Immuno-Inflammatory Alliance Regulatory	Dec. 31, 2012 Immuno-Inflammatory Alliance Commercialization	Dec. 31, 2012 Immuno-Inflammatory Alliance Fourth Micro RNA Targets	Dec. 31, 2012 Immuno-Inflammatory Alliance Development Candidate	Dec. 31, 2012 Immuno-Inflammatory Alliance Minimum	Dec. 31, 2012 Immuno-Inflammatory Alliance Maximum	Dec. 31, 2012 HCV Alliance	Dec. 31, 2012 HCV Alliance Preclinical	Dec. 31, 2012 HCV Alliance Clinical	Dec. 31, 2012 HCV Alliance Regulatory	Dec. 31, 2012 HCV Alliance Commercialization Milestones	Dec. 31, 2012 HCV Alliance Phase One Clinical Trial	Dec. 31, 2012 HCV Alliance Phase Two Clinical Trial	Dec. 31, 2012 HCV Alliance Proof-of-Concept Trial	Dec. 31, 2012 Biogen Idec	Dec. 31, 2012 Biogen Idec Research Tax Credit Carryforward	Dec. 31, 2012 Biogen Idec Identification Of Micro Rna Biomarker	Dec. 31, 2012 Biogen Idec Validation Of Micro Rna Biomarker	Dec. 31, 2012 Biogen Idec Longitudinal Study Of Patient Samples On Ms Therapy
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue recognition period							8 years
Revenue through milestone payments				$ 5,000,000	$ 15,000,000		$ 509,000,000	$ 10,000,000	$ 5,000,000	$ 2,500,000	$ 129,000,000	$ 370,000,000			$ 432,500,000	$ 500,000	$ 15,500,000	$ 87,000,000	$ 150,000,000	$ 180,000,000	$ 500,000	$ 5,000,000			$ 144,000,000	$ 5,000,000	$ 29,000,000	$ 50,000,000	$ 60,000,000	$ 4,000,000	$ 5,000,000	$ 20,000,000		$ 1,300,000	$ 250,000	$ 500,000	$ 500,000
Royalties based on percentage of net sales													10.00%	20.00%									10.00%	20.00%
Number of collaborative areas granted							3
Collaboration and license agreement date							August 2012																										August 2012
Received upfront payment							3,000,000																										800,000
Expected term of research and development plan						4 years
Common stock issue		36,000					25,000,000
Common stock issued		35,831,808	153,184				6,250,000
Price per share		$ 0.001	$ 0.001				$ 4.00
Number of days during which AstraZeneca could not sell, transfer, make any short sale of, or grant any option for the sale of any common stock							365 days
Restricting common stock valuation measurement period							1 year
Restricting common stock valuation, discounted amount							4.3
Upfront payment being recognized as revenue																																	800,000
Revenue recognition period																																	2 years
Issued promissory note	$ 5,000,000	$ 5,000,000
Conversion to common stock	1,256,232	1,256,232
Conversion rate	$ 4.00	$ 4.00