Leases - Additional Information (Detail)	1 Months Ended								9 Months Ended
	Nov. 30, 2019 USD ($) ft²	Oct. 31, 2019 USD ($)	May 31, 2019	Nov. 30, 2018 USD ($) ft²	Apr. 30, 2018 USD ($)	May 31, 2016 USD ($)	Jan. 31, 2016 USD ($)	Mar. 31, 2015	Sep. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Other Commitments [Line Items]
Property and equipment, net									$ 28,287,000		$ 28,702,000
Financing lease obligation											$ 5,854,000
Operating lease liabilities									(1,832,000)
Decrease in right-of-use assets									(1,712,000)
Operating lease right-of-use assets									5,905,000
Operating lease liabilities									$ 2,506,000
ASU 2016-02 [Member]
Other Commitments [Line Items]
Operating lease right-of-use assets										$ 7,400,000
Rockville, Maryland [Member]
Other Commitments [Line Items]
Operating leases expiration year									2022
9800 Medical Center Drive Lease [Member] | Subsequent Event
Other Commitments [Line Items]
Tenant improvement allowance to be received	$ 19,500,000
Number of square feet to be constructed | ft²	38,000
Additional Lease Rental Payments	$ 22,600,000
9800 Medical Center Drive Lease [Member] | Leasehold Improvements [Member]
Other Commitments [Line Items]
Property and equipment, net									$ 5,400,000
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]
Operating lease expiration year									2019-08
Operating lease liabilities									$ 400,000
Decrease in right-of-use assets									400,000
Operating lease right-of-use assets									0
Operating lease liabilities									$ 0
400 Madison Avenue [Member] | 400 Madison Lease [Member] | Lease Amendment [Member] | Subsequent Event
Other Commitments [Line Items]
Tenant improvement allowance to be received		$ 700,000
Operating leases expiration term		7 years 6 months