REVENUES - Schedule of Revenue from Contract with Customer (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | $ 1,860,600 | | $ 4,138,900 | |
Operating Revenues | 1,853,856 | $ 1,762,811 | 4,141,818 | $ 3,867,946 |
Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 930,800 | | 2,201,300 | |
Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 705,700 | | 1,466,000 | |
Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 110,600 | | 220,100 | |
Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 1,747,100 | | 3,887,400 | |
Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 55,400 | | 120,500 | |
Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 47,200 | | 124,300 | |
Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 22,200 | | 43,200 | |
Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 11,300 | | 36,500 | |
Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | (11,900) | | (5,400) | |
Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 5,200 | | 8,300 | |
The Connecticut Light and Power Company | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 694,892 | 666,558 | 1,479,875 | 1,398,867 |
NSTAR Electric Company | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 690,737 | 704,702 | 1,460,865 | 1,438,510 |
Public Service Company of New Hampshire | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 235,146 | 230,383 | 502,497 | 483,541 |
Operating Segments | Electric Distribution | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 1,550,100 | | 3,293,500 | |
Operating Revenues | 1,539,900 | 1,275,900 | 3,294,500 | 2,677,000 |
Operating Segments | Electric Distribution | Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 794,400 | | 1,788,700 | |
Operating Segments | Electric Distribution | Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 622,000 | | 1,233,400 | |
Operating Segments | Electric Distribution | Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 88,400 | | 169,900 | |
Operating Segments | Electric Distribution | Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 1,504,800 | | 3,192,000 | |
Operating Segments | Electric Distribution | Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Electric Distribution | Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 34,200 | | 92,700 | |
Operating Segments | Electric Distribution | Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 18,400 | | 35,300 | |
Operating Segments | Electric Distribution | Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 7,300 | | 26,500 | |
Operating Segments | Electric Distribution | Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | (14,400) | | (5,700) | |
Operating Segments | Electric Distribution | Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 4,200 | | 6,700 | |
Operating Segments | Natural Gas Distribution | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 205,600 | | 631,700 | |
Operating Revenues | 206,500 | 186,000 | 631,400 | 589,600 |
Operating Segments | Natural Gas Distribution | Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 104,100 | | 352,900 | |
Operating Segments | Natural Gas Distribution | Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 70,000 | | 204,700 | |
Operating Segments | Natural Gas Distribution | Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 23,500 | | 53,100 | |
Operating Segments | Natural Gas Distribution | Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 197,600 | | 610,700 | |
Operating Segments | Natural Gas Distribution | Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Natural Gas Distribution | Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 12,100 | | 29,900 | |
Operating Segments | Natural Gas Distribution | Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | (600) | | (900) | |
Operating Segments | Natural Gas Distribution | Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 3,500 | | 8,000 | |
Operating Segments | Natural Gas Distribution | Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 100 | | (1,700) | |
Operating Segments | Natural Gas Distribution | Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 800 | | 1,400 | |
Operating Segments | Electric Transmission | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 314,000 | | 630,700 | |
Operating Revenues | 317,400 | 324,600 | 622,400 | 641,500 |
Operating Segments | Electric Transmission | Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Electric Transmission | Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Electric Transmission | Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Electric Transmission | Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Electric Transmission | Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 310,800 | | 624,500 | |
Operating Segments | Electric Transmission | Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Electric Transmission | Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 3,200 | | 6,200 | |
Operating Segments | Electric Transmission | Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Electric Transmission | Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 3,400 | | (8,300) | |
Operating Segments | Electric Transmission | Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 0 | | 0 | |
Operating Segments | Water Distribution | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 51,400 | | 95,200 | |
Operating Revenues | 53,500 | 0 | 98,000 | 0 |
Operating Segments | Water Distribution | Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 32,300 | | 59,700 | |
Operating Segments | Water Distribution | Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 15,700 | | 30,000 | |
Operating Segments | Water Distribution | Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 1,100 | | 2,100 | |
Operating Segments | Water Distribution | Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 49,100 | | 91,800 | |
Operating Segments | Water Distribution | Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Water Distribution | Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 900 | | 1,700 | |
Operating Segments | Water Distribution | Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 1,900 | | 3,700 | |
Operating Segments | Water Distribution | Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 500 | | 2,000 | |
Operating Segments | Water Distribution | Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 1,900 | | 2,600 | |
Operating Segments | Water Distribution | Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 200 | | 200 | |
Operating Segments | Other | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 237,000 | | 467,900 | |
Operating Revenues | 237,000 | | 467,900 | |
Operating Segments | Other | Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Other | Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Other | Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Other | Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Other | Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 12,600 | | 22,700 | |
Operating Segments | Other | Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Other | Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 224,400 | | 445,200 | |
Operating Segments | Other | Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Operating Segments | Other | Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 0 | | 0 | |
Operating Segments | Other | Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 0 | | 0 | |
Operating Segments | The Connecticut Light and Power Company | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 798,900 | | 1,697,100 | |
Operating Segments | The Connecticut Light and Power Company | Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 387,100 | | 870,400 | |
Operating Segments | The Connecticut Light and Power Company | Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 220,700 | | 443,200 | |
Operating Segments | The Connecticut Light and Power Company | Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 36,500 | | 72,300 | |
Operating Segments | The Connecticut Light and Power Company | Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 644,300 | | 1,385,900 | |
Operating Segments | The Connecticut Light and Power Company | Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 139,900 | | 290,600 | |
Operating Segments | The Connecticut Light and Power Company | Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 10,600 | | 21,000 | |
Operating Segments | The Connecticut Light and Power Company | Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 8,300 | | 16,200 | |
Operating Segments | The Connecticut Light and Power Company | Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 4,200 | | (16,600) | |
Operating Segments | NSTAR Electric Company | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 792,500 | | 1,648,000 | |
Operating Segments | NSTAR Electric Company | Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 292,500 | | 656,700 | |
Operating Segments | NSTAR Electric Company | Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 325,500 | | 640,000 | |
Operating Segments | NSTAR Electric Company | Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 30,900 | | 59,000 | |
Operating Segments | NSTAR Electric Company | Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 648,900 | | 1,355,700 | |
Operating Segments | NSTAR Electric Company | Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 120,900 | | 239,500 | |
Operating Segments | NSTAR Electric Company | Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 13,400 | | 38,200 | |
Operating Segments | NSTAR Electric Company | Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 9,300 | | 18,300 | |
Operating Segments | NSTAR Electric Company | Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | (3,700) | |
Operating Segments | Public Service Company of New Hampshire | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 273,800 | | 581,800 | |
Operating Segments | Public Service Company of New Hampshire | Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 114,800 | | 261,600 | |
Operating Segments | Public Service Company of New Hampshire | Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 76,200 | | 151,100 | |
Operating Segments | Public Service Company of New Hampshire | Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 21,000 | | 38,600 | |
Operating Segments | Public Service Company of New Hampshire | Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 212,000 | | 451,300 | |
Operating Segments | Public Service Company of New Hampshire | Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 50,100 | | 94,300 | |
Operating Segments | Public Service Company of New Hampshire | Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 10,900 | | 34,900 | |
Operating Segments | Public Service Company of New Hampshire | Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 3,900 | | 7,500 | |
Operating Segments | Public Service Company of New Hampshire | Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 3,100 | | (6,200) | |
Segment Reconciling Items [Member] | The Connecticut Light and Power Company | Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 1,000 | | (4,100) | |
Segment Reconciling Items [Member] | The Connecticut Light and Power Company | Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 2,400 | | 3,300 | |
Segment Reconciling Items [Member] | NSTAR Electric Company | Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | (6,900) | | (200) | |
Segment Reconciling Items [Member] | NSTAR Electric Company | Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 1,600 | | 2,700 | |
Segment Reconciling Items [Member] | Public Service Company of New Hampshire | Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | (5,000) | | (9,600) | |
Segment Reconciling Items [Member] | Public Service Company of New Hampshire | Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 200 | | 600 | |
Eliminations | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | (497,500) | | (980,100) | |
Operating Revenues | (500,400) | $ (240,700) | (972,400) | $ (493,600) |
Eliminations | Residential | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Eliminations | Commercial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | (2,000) | | (2,100) | |
Eliminations | Industrial | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | (2,400) | | (5,000) | |
Eliminations | Total Retail Tariff Sale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | (4,400) | | (7,100) | |
Eliminations | Transmission Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | (268,000) | | (526,700) | |
Eliminations | Wholesale Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Eliminations | Other Revenue from Contracts with Customers | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | (225,100) | | (446,300) | |
Eliminations | Reserve for Revenue Subject to Refund | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Revenue from Contracts with Customers | 0 | | 0 | |
Eliminations | Alternative Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | (2,900) | | 7,700 | |
Eliminations | Other Revenue | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | 0 | | 0 | |
Consolidations Eliminations | The Connecticut Light and Power Company | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | (107,400) | | (216,400) | |
Consolidations Eliminations | NSTAR Electric Company | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | (96,500) | | (189,600) | |
Consolidations Eliminations | Public Service Company of New Hampshire | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Operating Revenues | $ (33,900) | | $ (70,300) | |