Commitments and contingencies (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($)
Auburn, Washington
Rent Expense [Line Items]
Rent expense	$ 39,000
Lease Expiration Date	Aug. 31, 2018
Baton Rouge, Louisiana
Rent Expense [Line Items]
Rent expense	$ 23,000
Lease Expiration Date	May 31, 2019
Champaign, Illinois
Rent Expense [Line Items]
Rent expense	$ 504,000
Lease Expiration Date	Dec. 31, 2018
Charlotte, North Carolina
Rent Expense [Line Items]
Rent expense	$ 26,000
Lease Expiration Date	May 31, 2019
Escondido, California
Rent Expense [Line Items]
Rent expense	$ 36,000
Lease Expiration Period Description	Month to Month
Hampstead, New Hampshire
Rent Expense [Line Items]
Rent expense	$ 52,000
Lease Expiration Date	Aug. 31, 2022
Nashville, Tennessee
Rent Expense [Line Items]
Rent expense	$ 173,000
Lease Expiration Date	Mar. 31, 2018
Ontario, California
Rent Expense [Line Items]
Rent expense	$ 90,000
Lease Expiration Date	Dec. 31, 2018
Pearl River, New York
Rent Expense [Line Items]
Rent expense	$ 150,000
Lease Expiration Date	Dec. 31, 2021
Pottsboro, Texas
Rent Expense [Line Items]
Rent expense	$ 6,000
Lease Expiration Date	Aug. 31, 2017
Catano, Puerto Rico
Rent Expense [Line Items]
Rent expense	$ 124,000
Lease Expiration Date	Dec. 31, 2020
Stony Point, New York
Rent Expense [Line Items]
Rent expense	$ 90,000
Lease Expiration Date	Jun. 30, 2021
Tulsa, Oklahoma
Rent Expense [Line Items]
Rent expense	$ 27,000
Lease Expiration Date	Dec. 31, 2017
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	$ 782,000
Lease Expiration Date	Jul. 31, 2018
Riverside, California [Member]
Rent Expense [Line Items]
Rent expense	$ 27,000
Lease Expiration Period Description	Month to Month
Smyrna, Georgia [Member]
Rent Expense [Line Items]
Rent expense	$ 446,000
Lease Expiration Date	Jul. 31, 2030