Commitments and Contingencies - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended			3 Months Ended		6 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2014 sqft	Jun. 30, 2014 sqft	Jun. 30, 2014 Collateralized letters of credit [Member]	Dec. 31, 2013 Collateralized letters of credit [Member]	Mar. 31, 2014 Fifth Amendment To Lease Agreement [Member] sqft	Jun. 30, 2014 Fifth Amendment To Lease Agreement [Member]	Jun. 30, 2014 Development milestone payments [Member]	Mar. 31, 2014 Sales milestone payments [Member] Astellas Pharma Inc. [Member]	Jun. 30, 2014 Sales milestone payments [Member] Astellas Pharma Inc. [Member]	Dec. 31, 2013 Sales milestone payments [Member] Astellas Pharma Inc. [Member]
Commitments And Contingencies [Line Items]
Operating lease office space, in square feet	127,000	127,000			29,000
Tenant improvement allowance					$ 900,000
Outstanding letters of credit collateralized by restricted cash			12,000,000	9,900,000	2,100,000
Lease expiry period		June 2019
Minimum lease payments over the lease term						9,600,000
Aggregate milestone payments upon achievement of certain development and regulatory milestone events							2,800,000
Percentage of sublicensing income							10.00%	10.00%	10.00%
Royalty percentage on sales							4.00%
License and research agreement fee	12,000,000
Milestone payments eligible to earn									320,000,000
Milestone payments earned								25,000,000	25,000,000
Accrued payment to licensor										2,500,000
Coverage under its relevant insurance policies		$ 350,000