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Net Sales (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Revenue from Contract with Customer [Abstract] |  |  |
Customer Prepayment Current | $ 1,000 | $ 1,000 |
Revenue Recognized in the Current Period From Funds Collected in Advance from Customers in Earlier Periods | 1,000 | 1,000 | $ 3,000
Change in Net Sales Related To Adjustments To Rebates Or Business Development Fund Recognized in Revenue in a Prior Period | 2,000 | 1,000 | $ 3,000
Customer rebates | 23,641 | 22,682 |
Customer Rebates Current | $ 1,000 | $ 2,000 |

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- Definition Accrual related to business development funds as of the balance sheet date. + References No definition available. + Details
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Balance Type: | credit
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- Definition Change in Net Sales Related To Adjustments To Rebates Or Business Development Fund Recognized in Revenue in a Prior Period + References No definition available. + Details
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Namespace Prefix: | neu_
Data Type: | xbrli:monetaryItemType
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- Definition Current portion prepayments received from customers for goods and services to be provided in the future. + References No definition available. + Details
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Namespace Prefix: | neu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Total of the carrying values as of the balance sheet date of the provision for customer rebates. + References No definition available. + Details
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Namespace Prefix: | neu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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X
- Definition Revenue Recognized in the current period From Funds Collected in Advance from Customers in Earlier Periods + References No definition available. + Details
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Namespace Prefix: | neu_
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Balance Type: | credit
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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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