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Alabama State Industrial Development Authority Financing and Economic Incentives - Additional Information (Detail) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
	Jan. 02, 2020	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2008	Jan. 13, 1995
Debt Instrument [Line Items]
Repayment of bond		$ 24,600
Taxable Revenue Bonds [Member]
Debt Instrument [Line Items]
Proceeds from state industrial development authority issued taxable bonds loaned to ADTRAN			$ 24,600	$ 50,000	$ 20,000
Percentage of interest on bond			2.00%
Maturity date of bond			Jan. 01, 2020
Repayment of bond	$ 24,600
Certificate of deposit			$ 25,600