License Agreements - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended								9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended					9 Months Ended		12 Months Ended		1 Months Ended	12 Months Ended									1 Months Ended
	Apr. 30, 2007	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012	Mar. 31, 2011	Dec. 31, 2011 Licensing Agreements	Dec. 31, 2010 Licensing Agreements	Dec. 31, 2012 Licensing Agreements	Mar. 31, 2011 Licensing Agreements	Mar. 31, 2010 Licensing Agreements License Fees	Mar. 31, 2011 Licensing Agreements Sub License Fees	Mar. 31, 2010 Licensing Agreements Sub License Fees	Dec. 31, 2011 Licensing Agreements Milestone Payable	Dec. 31, 2012 Families of Spinal Muscular Atrophy Licensing Agreements	Mar. 31, 2010 Families of Spinal Muscular Atrophy Licensing Agreements License Fees	Mar. 31, 2010 Families of Spinal Muscular Atrophy Licensing Agreements Sub License Fees	Apr. 30, 2011 Families of Spinal Muscular Atrophy Licensing Agreements Milestone Payable	Dec. 31, 2012 Scripps Research Institute Licensing Agreements	Dec. 31, 2012 Pfizer Incorporation Nonsoftware License Arrangement	Dec. 31, 2012 Pfizer Incorporation Nonsoftware License Arrangement Allocated Based on Selling Price of Deliverables	Dec. 31, 2012 Pfizer Incorporation Nonsoftware License Arrangement Transitional Services	Dec. 31, 2012 Pfizer Incorporation Nonsoftware License Arrangement Completion of Second Cohort Clinical Trial	Dec. 31, 2012 Pfizer Incorporation Nonsoftware License Arrangement Clinical Trial Material	Dec. 31, 2012 Pfizer Incorporation Nonsoftware License Arrangement Clinical Milestone Events	Dec. 31, 2012 Pfizer Incorporation Nonsoftware License Arrangement First Commercial Sale Milestone Events	Dec. 31, 2012 Pfizer Incorporation Nonsoftware License Arrangement First Clinical Milestone Payment	Jan. 31, 2013 Pfizer Incorporation Nonsoftware License Arrangement Up-front Payment Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payment received under license agreement																																			$ 5,000,000
Total future milestone receivable																											65,000,000					30,000,000	35,000,000	2,000,000
License agreement, revenue recognized																											4,876,000	124,000	76,000	35,000	13,000
Estimated selling price used to allocate consideration																													600,000	275,000	105,000
License Agreement, initial license fee																										300,000
Total future milestone payments																										4,300,000
Common stock, shares issued																										87,464
Common stock, shares issued value																										300,000
Common stock, shares purchased through issuance of warrants	150,000																									150,000
Warrant term	7 years																									7 years
License Agreement termination, written notice period																										90 days
Research and development expenses		2,343,000	2,433,000	2,906,000	2,808,000	2,870,000	3,075,000	3,517,000	3,785,000	9,461,960	8,744,548	10,489,811	12,528,819	525,000	343,000	55,000	374,000	500,000	65,000	175,000	500,000		500,000	175,000	500,000
Research and development, financial obligation																$ 16,000,000						$ 16,000,000
License Agreement, expiration term																						10 years