Commitments and contingencies (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($)
Auburn Washington [Member]
Rent Expense [Line Items]
Rent expense	$ 54,000
Lease Expiration Date	Aug. 31, 2019
Baton Rouge Louisiana [Member]
Rent Expense [Line Items]
Rent expense	$ 24,000
Lease Expiration Date	May 31, 2019
Champaign, Illinois [Member]
Rent Expense [Line Items]
Rent expense	$ 610,000
Lease Expiration Date	Dec. 31, 2019
Charlotte, North Carolina [Member]
Rent Expense [Line Items]
Rent expense	$ 26,000
Lease Expiration Date	May 31, 2019
Escondido,California [Member]
Rent Expense [Line Items]
Rent expense	$ 163,000
Lease Expiration Date	Jun. 30, 2022
Hampstead, New Hampshire [Member]
Rent Expense [Line Items]
Rent expense	$ 52,000
Lease Expiration Date	Aug. 31, 2022
Nashville, Tennessee [Member]
Rent Expense [Line Items]
Rent expense	$ 164,000
Lease Expiration Date	Mar. 31, 2020
Long Beach, California [Member]
Rent Expense [Line Items]
Rent expense	$ 26,400
Lease Expiration Date	Feb. 29, 2020
Long Island City, New York [Member]
Rent Expense [Line Items]
Rent expense	$ 756,000
Lease Expiration Date	Jul. 31, 2021
Ontario, California [Member]
Rent Expense [Line Items]
Rent expense	$ 92,000
Lease Expiration Date	Dec. 31, 2021
Pearl River, New York [Member]
Rent Expense [Line Items]
Rent expense	$ 150,000
Lease Expiration Date	Dec. 31, 2021
Pottsboro, Texas [Member]
Rent Expense [Line Items]
Rent expense	$ 9,600
Lease Expiration Period Description	Month to
Riverside, California [Member]
Rent Expense [Line Items]
Rent expense	$ 27,000
Lease Expiration Period Description	Month
Smyrna, Georgia [Member]
Rent Expense [Line Items]
Rent expense	$ 465,000
Lease Expiration Date	Jul. 31, 2030
Stony Point, New York [Member]
Rent Expense [Line Items]
Rent expense	$ 101,000
Lease Expiration Date	Jun. 30, 2021
Tulsa, Oklahoma [Member]
Rent Expense [Line Items]
Rent expense	$ 27,000
Lease Expiration Period Description	Month to Month