Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended							9 Months Ended	12 Months Ended			38 Months Ended	106 Months Ended
	Nov. 30, 2016	May 31, 2016	Jan. 31, 2016	Sep. 30, 2015	Mar. 31, 2015	Nov. 30, 2014	Mar. 31, 2009	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2017
Other Commitments [Line Items]
Tenant improvement allowance received				$ 400,000
Restricted cash as collateral with financial institution									$ 225,000	$ 225,000		$ 225,000	$ 225,000
Operating Leases, Rent Expense, Net									1,800,000	1,800,000	$ 400,000
Total expenses under license agreement									1,703,000	861,000	1,405,000
Claims paid to date related to indemnification issues									0
Accruals or expenses related to indemnification issues									0	0
European Patent Office Proceeding [Member]
Other Commitments [Line Items]
Liability related to third party opposition									$ 0			0	0
The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Percentage of ownership interest issued as consideration for license agreement									24.50%
Accrued expenses										200,000
The Trustees of the University of Pennsylvania [Member] | Maximum [Member]
Other Commitments [Line Items]
Accrued expenses									$ 100,000			100,000	100,000
The Trustees of the University of Pennsylvania [Member] | License Fees [Member]
Other Commitments [Line Items]
Related party transaction expense									444,000	574,000	866,000
The Trustees of the University of Pennsylvania [Member] | License Fees [Member] | Sublicense Fees [Member]
Other Commitments [Line Items]
Related party transaction expense								$ 400,000	207,000	430,000	703,000
The Trustees of the University of Pennsylvania [Member] | License Fees [Member] | Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Related party transaction expense								400,000	4,000	11,000	9,000
The Trustees of the University of Pennsylvania [Member] | License Fees [Member] | Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Related party transaction expense								400,000	233,000	133,000	154,000
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Percentage of ownership interest issued as consideration for license agreement							19.90%
Accrued expenses									300,000	400,000		300,000	300,000
Related party transaction expense									2,008,000	1,034,000	1,281,000
Milestone payment obligation							$ 1,500,000
GlaxoSmithKline LLC [Member] | Sublicense Fees [Member]
Other Commitments [Line Items]
Related party transaction expense								700,000	997,000	430,000	703,000
GlaxoSmithKline LLC [Member] | Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Related party transaction expense								700,000	2,000	7,000	5,000
GlaxoSmithKline LLC [Member] | Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Related party transaction expense								$ 700,000	509,000	597,000	573,000
GlaxoSmithKline LLC [Member] | Milestone Fees [Member]
Other Commitments [Line Items]
Related party transaction expense									500,000				500,000
Regents of the University of Minnesota [Member]
Other Commitments [Line Items]
Total expenses under license agreement									100,000	100,000
Amounts due under agreement									$ 100,000			100,000	$ 100,000
Regents of the University of Minnesota [Member] | Maximum [Member]
Other Commitments [Line Items]
Up-front fee paid and patent maintenance expenses reimbursed						$ 100,000
Up-front fee paid	$ 100,000
Total expenses under license agreement											$ 100,000
Amounts due under agreement										$ 100,000
Regents of the University of Minnesota [Member] | Milestone Fees [Member] | Maximum [Member]
Other Commitments [Line Items]
Related party transaction expense												$ 100,000
Regents of the University of Minnesota [Member] | Annual License Maintenance Fee [Member] | Maximum [Member]
Other Commitments [Line Items]
Contractual obligation						100,000
Regents of the University of Minnesota [Member] | Royalties And Sublicense Fees [Member] | Maximum [Member]
Other Commitments [Line Items]
Contractual obligation						$ 100,000
Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease commenced date			2016-02		2015-04
Lease term									September 2015, November 2015 and July 2017: Ultimately extend the term of the lease to September 2021.
Options to extend the additional lease term				6 years
Tenant improvement allowance received			$ 800,000
Lease period			7 years 6 months
Lease expiration date			Sep. 30, 2023
Options to extend the additional lease term			5 years
Monthly lease payments commencement period			2016-09
New York, New York [Member]
Other Commitments [Line Items]
Lease commenced date		2016-07
Lease expiration date		Oct. 31, 2020
Monthly lease payments commencement period		2016-10
Lease expiration term		51 months
Letters of credit		$ 200,000