License and Collaboration Agreements - Additional Information (Detail)							1 Months Ended				3 Months Ended		6 Months Ended
	Feb. 28, 2015 USD ($)	Feb. 24, 2015 USD ($)	Feb. 23, 2015 USD ($)	May. 29, 2014 USD ($)	Dec. 12, 2013 USD ($)	Apr. 11, 2013 USD ($)	Apr. 30, 2015 USD ($)	Dec. 31, 2014 USD ($) $ / shares	Aug. 31, 2012	Feb. 28, 2011 USD ($) Installment $ / shares shares	Jun. 30, 2015 USD ($) $ / shares	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) $ / shares	Jun. 30, 2014 USD ($)	Jul. 31, 2015 USD ($)	Nov. 30, 2012 USD ($)
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development expense											$ 16,588,000	$ 7,424,000	$ 25,849,000	$ 9,516,000
Common stock, par value | $ / shares								$ 0.001			$ 0.001		$ 0.001
Expenses in connection with license agreement					$ 30,000						$ 253,967	(7,575)	$ 339,345	152,969
Expenses on agreement payments					350,000
Payments to milestone					3,900,000
Milestone payments on due on marketing approval					3,000,000
Milestone payments reduced					2,700,000
Other assets								$ 127,000			9,226,000		9,226,000
UCLA License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development expense										$ 10,000
Repayment on patent										$ 166,000
Payment expenses terms monthly wise | Installment										24
Number of common shares issued for research and development | shares										27,400
Common stock, par value | $ / shares										$ 0.001
Value of stock options exercised										$ 10,412
Milestone payments										2,200,000			$ 0	0
Due on milestone payments										$ 2,000,000
C R A D A
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development arrangement, quarterly payment		$ 750,000	$ 250,000
Research and development arrangement, description and terms													The CRADA has a five-year term commencing August 31, 2012 and expiring on August 30, 2017.
Research and development arrangement, term									5 years
Total payments for research and development services agreement											750,000	$ 250,000	$ 1,166,667	$ 500,000
2013 NIH License
Research And Development Arrangement Contract To Perform For Others [Line Items]
Repayment on patent						$ 58,000
Expenses in connection with license agreement						200,000
Payments for royalties						20,000
Payments to benchmark						8,100,000		0			0		0
Benchmark payments due on marketing approval						6,000,000
First benchmark payments						50,000
Benchmark payments based on net sales						7,000,000
Net sales of licensed products						$ 1,000,000,000
Cabaret License
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development expense for agreement					$ 25,000
Termination fees																$ 500,000
Cabaret License | Amgen Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Sublicense Fee							$ 13,800,000				800,000		1,500,000
Sublicense applicable taxes payment							$ 1,800,000
Deferred asset current											3,400,000		3,400,000
Other assets											8,900,000		8,900,000
May 2014 NIH License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Repayment on patent				$ 30,000
Payments for royalties				20,000
Payments to benchmark				4,000,000
Benchmark payments due on marketing approval				3,000,000
Benchmark payments based on net sales				1,000,000,000
Net sales of licensed products				7,000,000
Research and development expense for agreement				150,000
Payments to benchmark				$ 50,000
December 2014 NIH License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Repayment on patent								42,000
Payments for royalties								20,000
Payments to benchmark								6,000,000
Benchmark payments due on marketing approval								5,000,000
Net sales of licensed products								7,000,000
Research and development expense for agreement	$ 350,000
Payments to benchmark								$ 50,000
Amgen Inc.
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payments	525,000,000
Upfront payment received	$ 60,000,000										54,100,000		54,100,000
Deferred revenue recognized											4,400,000		7,300,000
Collaboration Agreement with bluebird bio
Research And Development Arrangement Contract To Perform For Others [Line Items]
Collaboration expenses													0
Leukemia and Lymphoma Society, Inc.
Research And Development Arrangement Contract To Perform For Others [Line Items]
Development and commercialization agreement											$ 2,500,000		$ 2,500,000
Leukemia and Lymphoma Society, Inc. | Subsequent Event
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront payment received															$ 500,000