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Quarterly Results of Operations - Quarterly Financial Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Quarterly results of operations                      
Net sales $ 30,000 $ 28,400 $ 23,800 $ 32,300 $ 29,600 $ 29,700 $ 33,700 $ 31,200 $ 114,537 $ 124,243 $ 118,217
Gross margin 7,700 7,300 7,400 10,400 9,000 9,400 11,000 9,600 32,848 38,963 38,271
Net income (loss) 4,600 4,100 4,900 2,500 6,700 (1,000) 6,600 15,700 16,103 28,035 (39,013)
Amounts attributable to NL stockholders:                      
Net income (loss) $ 4,300 $ 3,900 $ 4,600 $ 1,900 $ 6,300 $ (1,600) $ 5,900 $ 15,200 $ 14,680 $ 25,844 $ (41,017)
Income (loss) per common share $ 0.09 $ 0.08 $ 0.09 $ 0.04 $ 0.13 $ (0.03) $ 0.12 $ 0.31 $ 0.30 $ 0.53 $ (0.84)