Commitments and Contingencies - Additional Information (Detail)	3 Months Ended			6 Months Ended
	Jun. 30, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)	Jun. 30, 2015 USD ($) ft²
Commitments And Contingencies [Line Items]
Operating lease office space, in square feet | ft²	143,000			143,000
Lease expiry period				June 2019
Percentage of sublicensing income				10.00%
Upfront and setup fees			$ 12,000,000
Total consideration under current statement of work		$ 14,500,000
Partial payment consideration under current statement of work				$ 1,000,000
CureTech Ltd [Member]
Commitments And Contingencies [Line Items]
Milestone payment				5,000,000
CureTech Ltd [Member] | MDV 9300 [Member]
Commitments And Contingencies [Line Items]
Upfront and setup fees	$ 200,000	$ 3,000,000
Manufacturing services and supply agreement period		3 years
Milestone payment				5,000,000
Funding for clinical trial materials				19,300,000
Clinical trial materials amount paid				$ 6,000,000
Funding period for clinical materials				3 years
Collateralized letters of credit [Member]
Commitments And Contingencies [Line Items]
Outstanding letters of credit collateralized by restricted cash	13,300,000	$ 11,800,000		$ 13,300,000
Current asset [Member] | Collateralized letters of credit [Member]
Commitments And Contingencies [Line Items]
Outstanding letters of credit collateralized by restricted cash	600,000	200,000		600,000
Long-term assets [Member] | Collateralized letters of credit [Member]
Commitments And Contingencies [Line Items]
Outstanding letters of credit collateralized by restricted cash	$ 12,700,000	$ 11,600,000		$ 12,700,000
Sixth Amendment To Lease Agreement [Member]
Commitments And Contingencies [Line Items]
Operating lease office space, in square feet | ft²	16,000			16,000
Tenant improvement allowance				$ 300,000
Outstanding letters of credit collateralized by restricted cash	$ 1,600,000			1,600,000
Minimum lease payments over the lease term	6,100,000			6,100,000
Development milestone payments [Member]
Commitments And Contingencies [Line Items]
Aggregate milestone payments upon achievement of certain development and regulatory milestone events	$ 2,800,000			$ 2,800,000
Percentage of sublicensing income				10.00%
Royalty percentage on sales				4.00%
Sales milestone payments [Member] | Astellas Pharma Inc. [Member]
Commitments And Contingencies [Line Items]
Percentage of sublicensing income				10.00%
Eligible to receive milestone payments				$ 320,000,000
Sales milestone payments under collaborative agreement				75,000,000
Payment related to sales milestones				$ 7,500,000