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REGULATION - REGULATORY FILINGS (Details) |  |  |  |  |  |  | 1 Months Ended | 2 Months Ended | 4 Months Ended | 12 Months Ended
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Apr. 01, 2018 USD ($) | Nov. 01, 2017 USD ($) | Sep. 22, 2017 USD ($) | Jul. 20, 2017 USD ($) | Oct. 01, 2016 USD ($) | Nov. 01, 2015 | Sep. 30, 2017 USD ($) station | Sep. 30, 2016 USD ($) | Oct. 31, 2015 USD ($) | Sep. 30, 2015 USD ($) | Jul. 31, 2014 USD ($) project | Oct. 31, 2012 USD ($) | Jun. 30, 2012 USD ($) | Feb. 29, 2016 USD ($) | Feb. 29, 2016 USD ($) | Sep. 30, 2017 USD ($) station | Sep. 30, 2010 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Return on equity |  |  |  |  | 9.75% |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved equity capital structure, percentage |  |  |  |  | 52.50% |  |  |  |  |  |  |  |  |  |  |  |
Composite rate of depreciation |  |  |  |  | 2.40% |  |  |  |  |  |  |  |  |  |  |  |
Program recovery term |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |
Tax charge resulting from a change in deductibility of federal subsidies |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,400,000
Regulatory assets, noncurrent |  |  |  |  |  |  | $ 375,919,000 | $ 441,294,000 |  |  |  |  |  |  |  | $ 375,919,000 |
Number of NGV stations opened to the public | station |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  | 3 |
Other Regulatory Asset |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory asset, maximum amount to be recorded annually |  |  |  |  |  |  |  | 700,000 |  |  |  |  |  |  |  |  |
Regulatory asset, expense benchmark |  |  |  |  |  |  |  | 1,400,000 |  |  |  |  |  |  |  |  |
Regulatory assets, threshold of recording regulatory liability when net liability exceeds amount |  |  |  |  |  |  |  | 1,000,000 |  |  |  |  |  |  |  |  |
Other Regulatory Asset, PIM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets, noncurrent |  |  |  |  |  |  | $ 3,800,000 |  |  |  |  |  |  |  |  | $ 3,800,000 |
Regulatory assets, amortization period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7 years |
September 2016 Base Rate Case |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease), amount |  |  |  |  | $ 45,000,000 |  |  |  |  |  |  |  |  |  |  |  |
June 2015 BGSS/CIP Filing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Customer bill credits |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 61,600,000 |  |
June 2016 BGSS/CIP Filing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Customer bill credits |  |  |  |  |  |  |  | 48,000,000 |  |  |  |  |  |  |  | $ 42,000,000 |
June 2016 BGSS/CIP Filing | Basic Gas Supply Service |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease), amount |  |  |  |  |  |  |  | (22,600,000) |  |  |  |  |  |  |  |  |
June 2016 BGSS/CIP Filing | Conservation Incentive Program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease), amount |  |  |  |  |  |  |  | 43,900,000 |  |  |  |  |  |  |  |  |
June 2017 BGSS/CIP Filing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenues |  |  | $ 3,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
June 2017 BGSS/CIP Filing | Conservation Incentive Program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease), amount |  |  | $ (16,200,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual Petition With BPU |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested base rate increase, amount |  |  |  | $ 4,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
SAFE Program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investments approved by the BPU |  |  |  |  | 200,000,000 |  |  |  |  |  |  | $ 130,000,000 |  |  |  |  |
Capital investments to be recovered approved by the Board of Public Utilities |  |  |  |  | $ 157,500,000 |  |  |  |  |  |  |  |  |  |  |  |
Capital investments approved by the BPU, period |  |  |  |  |  |  |  |  |  |  |  | 4 years |  |  |  |  |
SAVEGREEN |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investments approved by the BPU |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 219,300,000 |
Grants, rebates and loans provided to customers |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 149,700,000 |
Increase in regulatory funding obligations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,000,000 |
SAVEGREEN | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Return on equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.75% |
Public utilities, approved equity capital structure, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.69% |
Program recovery term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years |
Regulatory assets, amortization period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years |
SAVEGREEN | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Return on equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.30% |
Public utilities, approved equity capital structure, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.76% |
Program recovery term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 years |
Regulatory assets, amortization period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 years |
NJ RISE Program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease), amount |  |  |  |  |  |  |  |  | $ 390,000 |  |  |  |  |  |  |  |
Return on equity |  |  |  |  |  | 9.75% |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved equity capital structure, percentage |  |  |  |  |  | 6.74% |  |  |  |  |  |  |  |  |  |  |
Program recovery term |  |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |
Number of capital investment projects | project |  |  |  |  |  |  |  |  |  |  | 6 |  |  |  |  |  |
Originally filed petition for capital investments to Board of Public Utilities |  |  |  |  |  |  |  |  |  |  | $ 102,500,000 |  |  |  |  |  |
SAFE II and NJ RISE [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Return on equity |  |  |  |  | 9.75% |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved equity capital structure, percentage |  |  |  |  | 6.90% |  |  |  |  |  |  |  |  |  |  |  |
USF |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease), amount |  |  |  |  |  |  | $ (2,600,000) | $ 1,300,000 |  | $ (3,900,000) |  |  |  |  |  |  |
RAC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 9,400,000 |
Compressed Natural Gas Vehicle Refueling Stations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investments approved by the BPU |  |  |  |  |  |  |  |  |  |  |  |  | $ 10,000,000 |  |  |  |
SRL-Southern Reliability Link | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated cost of the SRL |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 180,000,000 |
SRL-Southern Reliability Link | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated cost of the SRL |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000,000 |
LNG-Liquefied Natural Gas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital investments to be recovered approved by the Board of Public Utilities |  |  |  |  | $ 36,500,000 |  |  |  |  |  |  |  |  |  |  |  |
NJNG | June 2015 BGSS/CIP Filing | Conservation Incentive Program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,100,000 |  |  |
Subsequent Event | SAVEGREEN |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease), amount |  | $ (3,900,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Scenario, Forecast | RAC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate, amount | $ 7,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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