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Regulatory matters (Details) - USD ($) | Jan. 17, 2017 | Jan. 10, 2017 | Dec. 28, 2016 | Dec. 21, 2016 | Dec. 12, 2016 | Dec. 02, 2016 | Nov. 23, 2016 | Nov. 21, 2016 | Oct. 27, 2016 | Oct. 14, 2016 | Sep. 06, 2016 | Aug. 12, 2016 | Jun. 10, 2016 | Jun. 01, 2016 | Apr. 29, 2016 | Mar. 09, 2016 | Feb. 10, 2016 | Jan. 05, 2016 | Jan. 01, 2016 | Sep. 30, 2015
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MNPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,600,000
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.40%
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,500,000 |
Interim rate increase (decrease), percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.40% |
Public utilities, approved rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  | $ 1,100,000 |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), percentage |  |  |  |  |  |  |  |  |  |  | 5.20% |  |  |  |  |  |  |  |  |
NDPSC-Renewable Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,100,000 |  |
Public utilities, requested return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |
NDPSC-Electric Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 9,700,000 |  |  |  |
Public utilities, requested return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |  |  |
NDPSC-Transmission Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,800,000 |  |  |
Public utilities, requested return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |  |
OPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,900,000 |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.80% |  |  |  |  |
Public utilities, approved rate increase (decrease), amount |  |  |  |  | $ 754,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
WUTC-Pipeline Replacement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, replacement cost recovery mechanism |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,600,000 |  |  |  |  |  |
Public utilities, requested additional revenue, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  |  |  |  |  |
Incremental pipeline replacement investment |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,400,000 |  |  |  |  |  |
Incremental operation and maintenance costs |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,200,000 |  |  |  |  |  |
Incremental pipeline recovery investment, allowed amount |  |  |  |  |  |  |  |  | $ 1,900,000 |  |  |  |  |  |  |  |  |  |  |
WYPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,200,000 |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  | 13.10% |  |  |  |  |  |  |
Settlement stipulation |  |  | $ 2,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement stipulation percent |  |  | 11.10% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
IPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  | $ 10,200,000 |  |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  | 4.10% |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amended, amount |  |  |  |  |  |  | $ 9,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |
NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  | $ 13,400,000 |  |  |  |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  | 6.60% |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  | $ 11,700,000 |  | $ 13,000,000 |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), percentage |  |  |  |  |  |  |  | 5.80% |  | 6.50% |  |  |  |  |  |  |  |  |  |
MTPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities gas and electric tax tracking adjustment |  |  |  |  |  | $ 814,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
MNPUC-Gas Cost Tariff [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, gas utility infrastructure cost tariff, requested |  |  |  | $ 456,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | MISO [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue requirement |  | $ 11,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | MTPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities gas tax tracking adjustment, approved | $ 474,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities electric tax tracking adjustment, approved | $ 251,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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