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Income Taxes - Schedule of Changes in Amount of Uncertain Tax Positions (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Unrecognized liabilities: |  |  |
Balance at the beginning of the period | $ 7.3 | $ 7.3 | $ 12.2
Change in federal tax rate | 0.0 | 0.0 | (4.9)
Balance at the end of the period | $ 7.3 | $ 7.3 | $ 7.3

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- Definition Unrecognized tax benefits change in federal tax rate. + References No definition available. + Details
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Namespace Prefix: | nl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 10B -URI http://asc.fasb.org/extlink&oid=116821951&loc=SL37586934-109318 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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Namespace Prefix: | us-gaap_
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