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Regulatory matters (Details) - USD ($) | Jul. 31, 2017 | Jul. 26, 2017 | Jul. 21, 2017 | Jun. 30, 2017 | Jun. 16, 2017 | May 31, 2017 | May 30, 2017 | Apr. 28, 2017 | Apr. 06, 2017 | Jan. 17, 2017 | Dec. 21, 2016 | Dec. 02, 2016 | Nov. 21, 2016 | Oct. 14, 2016 | Aug. 12, 2016 | Jun. 10, 2016 | Mar. 25, 2016 | Mar. 09, 2016 | Feb. 10, 2016 | Jan. 05, 2016
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WYPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,200,000 |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13.10% |  |  |  |
Rate increase approved |  |  |  |  |  |  |  |  | $ 2,700,000 |  |  |  |  |  |  |  |  |  |  |
Percent above current rates approved |  |  |  |  |  |  |  |  | 11.10% |  |  |  |  |  |  |  |  |  |  |
IPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 10,200,000 |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.10% |  |  |  |  |
Rate increase approved |  |  |  |  |  |  |  | $ 4,100,000 |  |  |  |  |  |  |  |  |  |  |  |
Percent above current rates approved |  |  |  |  |  |  |  | 1.60% |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amended, amount |  |  |  |  |  |  |  |  |  | $ 9,400,000 |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  |  |  |  | 9.50% |  |  |  |  |  |  |  |  |  |  |  |
MTPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities gas and electric tax tracking adjustment |  |  |  |  |  |  |  |  |  |  |  | $ 814,000 |  |  |  |  |  |  |  |
Public utilities gas tax tracking adjustment, approved |  |  |  |  |  |  |  |  |  | 474,000 |  |  |  |  |  |  |  |  |  |
Public utilities electric tax tracking adjustment, approved |  |  |  |  |  |  | $ 88,000 |  |  | $ 251,000 |  |  |  |  |  |  |  |  |  |
MNPUC-Gas Cost Tariff [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, gas utility infrastructure cost tariff, requested |  |  |  |  |  |  |  |  |  |  | $ 456,000 |  |  |  |  |  |  |  |  |
MTPSC-Phase 2 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,700,000 |  |  |
WUTC-Pipeline Replacement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, replacement cost recovery mechanism |  |  |  |  |  | $ 1,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, replacement cost recovery mechanism, percentage |  |  |  |  |  | 0.75% |  |  |  |  |  |  |  |  |  |  |  |  |  |
NDPSC-ADP [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated project cost |  |  |  | $ 85,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13,400,000 |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.60% |  |  |  |  |  |
Rate increase approved |  |  |  |  | $ 7,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent above current rates approved |  |  |  |  | 3.70% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 11,700,000 | $ 13,000,000 |  |  |  |  |  |
Interim rate increase (decrease), percent |  |  |  |  |  |  |  |  |  |  |  |  | 5.80% | 6.50% |  |  |  |  |  |
NDPSC-Renewable Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  | 9.65% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NDPSC-Electric Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  | 9.45% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NDPSC-Riders [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage |  |  |  |  | 9.45% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NDPSC-Riders [Member] | NDPSC-Renewable Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,100,000
Return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.50%
NDPSC-Riders [Member] | NDPSC-Electric Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 9,700,000 |  |
Return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |
NDPSC-Riders [Member] | NDPSC-Transmission Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,800,000 |
Return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |
Subsequent Event [Member] | NDPSC-Natural Gas [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  | $ 5,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent above current rates requested |  |  | 5.40% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  | $ 4,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), percent |  |  | 4.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | WUTC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested | $ 5,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent above current rates requested | 2.70% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities refund from current rates, percentage |  | 19.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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