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Rate and Regulatory Matters (Details)	Sep. 30, 2022 $ / dekatherm
Regulated Operations [Abstract]
Demand Rate Pending	0.37250
Commodity Rate Pending	0.00935

The commodity rate pending Federal Energy Regulatory Commission (FERC) approval.

No definition available.

Name:	nwp_CommodityRatePending
Namespace Prefix:	nwp_
Data Type:	srt-types:perUnitItemType
Balance Type:	na
Period Type:	instant

The demand rate pending Federal Energy Regulatory Commisssion (FERC) approval.

No definition available.

Name:	nwp_DemandRatePending
Namespace Prefix:	nwp_
Data Type:	srt-types:perUnitItemType
Balance Type:	na
Period Type:	instant

No definition available.

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Namespace Prefix:	us-gaap_
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Period Type:	duration