Leases - Additional Information (Detail)	1 Months Ended						3 Months Ended	6 Months Ended
	May 31, 2019 USD ($)	Nov. 30, 2018 USD ($) ft²	Apr. 30, 2018 USD ($)	May 31, 2016 USD ($)	Jan. 31, 2016 USD ($)	Mar. 31, 2015	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Other Commitments [Line Items]
Property and equipment, net							$ 27,330,000	$ 27,330,000		$ 28,702,000
Financing lease obligation										$ 5,854,000
Operating lease liabilities								(1,171,000)
Decrease in right-of-use assets								$ (1,156,000)
Rockville, Maryland [Member]
Other Commitments [Line Items]
Operating leases expiration year								2022
9800 Medical Center Drive Lease [Member] | Leasehold Improvements [Member]
Other Commitments [Line Items]
Property and equipment, net							4,800,000	$ 4,800,000
9800 Medical Center Drive Lease [Member] | ASU 2016-02 [Member] | Restatement Adjustment [Member]
Other Commitments [Line Items]
Property and equipment, net									$ (5,900,000)
Financing lease obligation									$ (5,900,000)
9800 Medical Center Drive Lease [Member] | Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease Expiration Term		16 years
Construction completion, description								The initial construction of the building is being conducted by the landlord and is expected to be completed in 2020
Tenant improvement allowance to be received		$ 15,300,000
Lease not yet commenced, option to extend the lease								true
Lease not yet commenced, additional lease term		10 years
Lease not yet commenced, option to terminate the lease								true
Lease not yet commenced, option to terminate the lease, description								the option to terminate the lease after 12 years from the delivery of the leased premises to the Company. If the Company elects to terminate the lease, it will be subject to a termination fee equal to the unamortized tenant improvement allowance, rent abatement and landlord commissions as of the termination date, bearing interest at 5% per annum, plus four months of base rent and operating expenses.
Minimum release term for option to terminate		12 years
Percentage of early termination fee		5.00%
Term of required base rent to be paid as termination fee		4 months
Frequency of periodic payment								Monthly payments under the 9800 Medical Center Drive Lease begin approximately 12 months from the delivery of the leased premises to the Company and escalate annually in accordance with the lease agreement.
Periodic payment term, from the delivery of leased premises		12 months
Letters of credit		$ 800,000
Number of square feet to be constructed | ft²		139,000
9712 Medical Center Drive [Member] | Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease term commenced date						2015-04
Lease term								In September 2015, November 2015, July 2017 and April 2018, the Company amended the 9712 Medical Center Drive Lease to include additional office and laboratory space at 9714 Medical Center Drive, and ultimately extend the term of the lease to September 2021.
Operating lease, existence of option to extend the lease								true
Additional lease term under option to extend			6 years
Tenant improvement allowance received			$ 400,000
9600 Blackwell Road [Member] | Rockville, Maryland [Member]
Other Commitments [Line Items]
Percentage of early termination fee					8.00%
Lease term commenced date					2016-02
Operating lease, existence of option to extend the lease								true
Additional lease term under option to extend					5 years
Tenant improvement allowance received					$ 800,000
Lease expiration date					Sep. 30, 2023
Required period to terminate lease after commencement period					67 months
400 Madison Avenue [Member] | 400 Madison Lease [Member]
Other Commitments [Line Items]
Letters of credit				$ 200,000
Operating leases commencement year				2016-07
Operating lease expiration year	2020-10
400 Madison Avenue [Member] | 400 Madison Lease [Member] | Lease Amendment [Member]
Other Commitments [Line Items]
Tenant improvement allowance to be received	$ 700,000
Operating lease expiration year								2019-08
Operating leases expiration term	7 years 6 months
Operating lease liabilities							400,000	$ 400,000
Decrease in right-of-use assets							$ 400,000	$ 400,000