Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		0 Months Ended	1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Nov. 01, 2014	Feb. 28, 2015 sqft
Other Commitments [Line Items]
Rent expense	$ 0.4	$ 0.2
St. Jude Children's Research Hospital [Member]
Other Commitments [Line Items]
Litigation settlement date	Apr. 04, 2015
Litigation expense reimbursement	$ 0.2	$ 0.5
Seattle lease [Member]
Other Commitments [Line Items]
Area of leased office and laboratory space	23,191
Office and laboratory lease expiration date	Jun. 27, 2017		Jun. 29, 2017
Lease termination description	The Company may terminate the lease agreement with 120 days' notice after March 31, 2016.
Additional area of leased office and laboratory space			17,841
Operating lease commencement date			2014-12
Notice period for termination			120 days
Bothell Lease [Member]
Other Commitments [Line Items]
Area of leased office and laboratory space				68,000
Lease termination description	The Company has the right to terminate the Bothell Lease effective as of any date after the second and on or before the seventh anniversary of the commencement of the Bothell Lease term, with 12-months' advance written notice and payment of an early termination fee equal to two years of rent and any unamortized leasing commissions paid by the landlord to any broker with respect to the initial term of the Bothell Lease.
Operating lease commencement date				2015-03
Notice period for termination				12 months
Initial term of lease				10 years
Option to extend term description	The Company will also have two options to extend the term of the Bothell Lease by five years each option, subject to a market-based rent escalation provision.