Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended	3 Months Ended						9 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2014	Sep. 30, 2014 sqft	Mar. 31, 2014 Fifth Amendment To Lease Agreement [Member] sqft	Sep. 30, 2014 Fifth Amendment To Lease Agreement [Member]	Sep. 30, 2014 Collateralized letters of credit [Member]	Dec. 31, 2013 Collateralized letters of credit [Member]	Sep. 30, 2014 Current asset [Member] Collateralized letters of credit [Member]	Sep. 30, 2014 Long-term assets [Member] Collateralized letters of credit [Member]	Sep. 30, 2014 Development milestone payments [Member]	Mar. 31, 2014 Sales milestone payments [Member] Astellas Pharma Inc. [Member]	Sep. 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	29,000
Tenant improvement allowance			$ 0.9
Outstanding letters of credit collateralized by restricted cash			2.1		12.0	9.9	0.4	11.6
Lease expiry period		June 2019
Minimum lease payments over the lease term				9.6
Aggregate milestone payments upon achievement of certain development and regulatory milestone events									2.8
Percentage of sublicensing income									10.00%	10.00%	10.00%
Royalty percentage on sales									4.00%
License and research agreement fee	12.0
Milestone payments eligible to earn											320.0
Milestone payments earned										25.0	25.0
Accrued payment to licensor												$ 2.5