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Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011 T
Commitments and Contingencies (Textual) [Abstract] |
Excess purchase obligation on additional tons of coal reserves | 100,000,000
Aggregate purchase price | $ 80.0

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- Details
Name: | nrp_CommitmentsAndContingenciesTextualAbstract
Namespace Prefix: | nrp_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The minimum amount the entity agreed to spend under the long-term purchase commitment. + References No definition available. + Details
Name: | us-gaap_LongTermPurchaseCommitmentAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Sets forth the number of units of goods that must be purchased within the specified time period of the agreement to avoid penalties or unfavorable effects on pricing, among other adverse consequences. + References No definition available. + Details
Name: | us-gaap_LongTermPurchaseCommitmentMinimumQuantityRequired
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration