Strategic Alliances and Collaborations - Additional Information (Detail) (USD $)			9 Months Ended		1 Months Ended	9 Months Ended								1 Months Ended				1 Months Ended	9 Months Ended			9 Months Ended												9 Months Ended									1 Months Ended	9 Months Ended							1 Months Ended		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Sanofi	Feb. 28, 2014 Sanofi	Feb. 28, 2014 Sanofi Private Placement	Sep. 30, 2014 Sanofi Minimum United States	Sep. 30, 2014 Sanofi Minimum Outside of the United States	Sep. 30, 2014 Sanofi Maximum United States	Sep. 30, 2014 Sanofi Maximum Outside of the United States	Sep. 30, 2014 Sanofi Proof-of-Concept Trial	Sep. 30, 2014 Sanofi Clinical	Sep. 30, 2014 Sanofi Regulatory and Commercialization Milestones	Sep. 30, 2014 Sanofi Common Stock Purchase Agreement	Feb. 28, 2014 Sanofi Development Commercialization And License Agreement	Jul. 31, 2013 Sanofi Development Commercialization And License Agreement	Jul. 31, 2013 Sanofi Research and Development	Sep. 30, 2014 Sanofi Research and Development	Jun. 30, 2013 GSK	Sep. 30, 2014 GSK	Sep. 30, 2014 GSK Development Commercialization And License Agreement	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Minimum	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Maximum	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Proof-of-Concept Trial	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Clinical	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Preclinical	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Regulatory	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Commercialization	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Fourth Micro RNA Targets	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Development Candidate	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Phase One Clinical Trial	Sep. 30, 2014 GSK Immuno-Inflammatory Alliance Phase Two Clinical Trial	Sep. 30, 2014 GSK HCV Alliance	Sep. 30, 2014 GSK HCV Alliance Minimum	Sep. 30, 2014 GSK HCV Alliance Maximum	Sep. 30, 2014 GSK HCV Alliance Proof-of-Concept Trial	Sep. 30, 2014 GSK HCV Alliance Clinical	Sep. 30, 2014 GSK HCV Alliance Preclinical	Sep. 30, 2014 GSK HCV Alliance Regulatory	Sep. 30, 2014 GSK HCV Alliance Phase One Clinical Trial	Sep. 30, 2014 GSK HCV Alliance Phase Two Clinical Trial	Sep. 30, 2014 GSK HCV Alliance Commercialization Milestones	Oct. 31, 2012 Astra Zeneca	Sep. 30, 2014 Astra Zeneca Agreement	Sep. 30, 2014 Astra Zeneca Minimum	Sep. 30, 2014 Astra Zeneca Maximum	Sep. 30, 2014 Astra Zeneca Clinical	Sep. 30, 2014 Astra Zeneca Preclinical	Sep. 30, 2014 Astra Zeneca Commercialization	Sep. 30, 2014 Astra Zeneca Common Stock Purchase Agreements Cspa	Aug. 31, 2014 Biogen Idec	Aug. 31, 2012 Biogen Idec	Sep. 30, 2014 Biogen Idec	Aug. 31, 2014 Biogen Idec Research Tax Credit Carryforward	Aug. 31, 2012 Biogen Idec Research Tax Credit Carryforward	Nov. 30, 2013 Biogen Idec August 2012 Collaboration and License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Initial upfront option payment														$ 2,500,000	$ 2,500,000	$ 25,000,000																											$ 3,000,000								$ 2,000,000	$ 800,000
Upfront payment creditable against future accounts receivables														1,250,000	1,250,000
Upfront payment non-creditable portion recognized															1,250,000
Deferred revenue				400,000									400,000				10,100,000			3,100,000																							4,300,000	3,400,000						3,400,000			1,700,000
Sale and issuance of common stock					1,303,780
Price per share					$ 7.67
Aggregate purchase price					10,000,000
Restriction period in which Alliances could not sell, transfer, make any short sale of, or grant any option for the sale of any common stock					12 months																																						365 days
Deferred revenue remaining recognition period			5 years																5 years																									2 years									10 months
Revenue through milestone payments										101,800,000	15,000,000	300,000,000									432,500,000			20,000,000	87,000,000	15,500,000	150,000,000	180,000,000	500,000	5,000,000	4,000,000	5,000,000	144,500,000			20,000,000	29,000,000	5,500,000	50,000,000	4,000,000	5,000,000	60,000,000		498,000,000			123,000,000	5,000,000	370,000,000					700,000	1,300,000	300,000
Royalties based on percentage of net sales						10.00%	10.00%	20.00%	20.00%													10.00%	20.00%											10.00%	20.00%										10.00%	20.00%
Number of collaborative areas granted																																												3
Collaboration and license agreement date																																												August 2012								August 2012
Expected term of research and development plan																																											4 years
Common stock issue	43,000	42,000																																										25,000,000
Common stock issued	43,439,562	41,787,326																																									6,250,000
Price per share																																											$ 4.00
Restricting common stock valuation measurement period																																											1 year
Deferred revenue recognition period																			8 years
Acceleration of remaining amortization of upfront payment																		$ 1,100,000																																	$ 100,000
Revenue recognition period																																																			1 year	2 years