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Regulatory matters (Details) - USD ($) | Oct. 27, 2016 | Oct. 14, 2016 | Oct. 06, 2016 | Sep. 06, 2016 | Aug. 12, 2016 | Jul. 07, 2016 | Jun. 15, 2016 | Jun. 10, 2016 | Jun. 01, 2016 | Apr. 29, 2016 | Mar. 09, 2016 | Feb. 10, 2016 | Jan. 05, 2016 | Jan. 01, 2016 | Dec. 01, 2015 | Sep. 30, 2015 | Jun. 30, 2015
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SDPUC-Electric [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,700,000
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 19.20%
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,700,000 |  |  |
Rate increase approved |  |  |  |  |  |  | $ 1,400,000 |  |  |  |  |  |  |  |  |  |
SDPUC-Natural Gas [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,500,000
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.10%
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,500,000 |  |  |
Rate increase approved |  |  |  |  |  |  | $ 1,200,000 |  |  |  |  |  |  |  |  |  |
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 |  |  |
Rate increase approved |  |  |  | $ 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), percent |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.40% |  |  |
Percent above current rates approved |  |  |  | 5.20% |  |  |  |  |  |  |  |  |  |  |  |  |
NDPSC-Renewable Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,100,000 |  |  |  |
Return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |  |  |
NDPSC-Electric Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  | $ 9,700,000 |  |  |  |  |  |
Return on equity, percentage |  |  |  |  |  |  |  |  |  |  | 10.50% |  |  |  |  |  |
NDPSC-Transmission Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  | $ 6,800,000 |  |  |  |  |
Return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |  |  |  |
WUTC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 10,500,000 |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.20% |  |
Rate increase approved |  |  |  |  |  | $ 4,000,000 |  |  |  |  |  |  |  |  |  |  |
OPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  | $ 1,900,000 |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  | 2.80% |  |  |  |  |  |  |
WUTC-Pipeline Replacement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Replacement cost recovery mechanism |  |  |  |  |  |  |  |  | $ 4,600,000 |  |  |  |  |  |  |  |
Percent of additional revenue requested |  |  |  |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |
Incremental pipeline recovery investment |  |  |  |  |  |  |  |  | $ 2,400,000 |  |  |  |  |  |  |  |
Incremental operation and maintenance costs |  |  |  |  |  |  |  |  | $ 2,200,000 |  |  |  |  |  |  |  |
WYPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  | $ 3,200,000 |  |  |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  | 13.10% |  |  |  |  |  |  |  |  |
IPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  | $ 10,200,000 |  |  |  |  |  |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  | 4.10% |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | OPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement |  |  | $ 754,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | WUTC-Pipeline Replacement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Incremental pipeline recovery investment, allowed amount | $ 1,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  | $ 13,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent above current rates requested |  | 6.60% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, interim rate increase (decrease), amount requested |  | $ 13,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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