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PROPERTY, PLANT AND EQUIPMENT (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment [Line Items]      
Property, plant and equipment $ 4,850,201,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService $ 4,534,074,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService  
Accumulated depreciation and amortization (1,556,481,000)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation (1,489,216,000)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation  
Net property, plant and equipment 3,293,720,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet 3,044,858,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet  
Regulated [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment 4,850,201,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
4,534,074,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
 
Accumulated depreciation and amortization (1,556,481,000)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
(1,489,216,000)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
 
Net property, plant and equipment 3,293,720,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
3,044,858,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
 
Average depreciation rates, minimum 2.00%ogs_AverageDepreciationRatesMinimum
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
2.00%ogs_AverageDepreciationRatesMinimum
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
2.00%ogs_AverageDepreciationRatesMinimum
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
Average depreciation rates, maximum 3.00%ogs_AverageDepreciationRatesMaximum
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
3.00%ogs_AverageDepreciationRatesMaximum
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
3.00%ogs_AverageDepreciationRatesMaximum
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
Interest costs capitalized 2,500,000us-gaap_InterestCostsCapitalized
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
1,300,000us-gaap_InterestCostsCapitalized
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
1,300,000us-gaap_InterestCostsCapitalized
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
Construction work in process expenditures incurred but not yet paid 7,000,000us-gaap_ConstructionInProgressExpendituresIncurredButNotYetPaid
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
10,500,000us-gaap_ConstructionInProgressExpendituresIncurredButNotYetPaid
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
12,000,000us-gaap_ConstructionInProgressExpendituresIncurredButNotYetPaid
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
Natural gas distribution pipelines and related equipment | Regulated [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment 3,909,881,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasDistributionEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
3,703,593,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasDistributionEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
 
Natural gas transmission pipelines and related equipment | Regulated [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment 450,810,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasTransmissionEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
430,042,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasTransmissionEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
 
General plant and other | Regulated [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment 418,157,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandBuildingsAndImprovementsMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
326,004,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandBuildingsAndImprovementsMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
 
Construction work in process | Regulated [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment $ 71,353,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
$ 74,435,000us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember