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Regulatory matters (Details) - USD ($) | Feb. 08, 2016 | Jan. 25, 2016 | Jan. 05, 2016 | Jan. 01, 2016 | Dec. 01, 2015 | Nov. 30, 2015 | Nov. 25, 2015 | Nov. 02, 2015 | Oct. 26, 2015 | Oct. 21, 2015 | Oct. 05, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Jun. 25, 2015 | Mar. 31, 2015
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OPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,600,000
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.10%
Settlement agreement |  |  |  |  |  |  |  | $ 600,000 |  |  |  |  |  |  |
Settlement agreement, percentage |  |  |  |  |  |  |  | 0.80% |  |  |  |  |  |  |
MTPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 11,800,000 |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.10% |
Amount of requested interim increase in annual rates |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 11,000,000 |
SDPUC-Electric [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,700,000 |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  | 19.20% |  |
SDPUC-Natural Gas [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,500,000 |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  | 3.10% |  |
MISO [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue requirement |  |  |  |  |  |  |  |  |  |  | $ 3,800,000 |  |  |  |
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 |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), percentage |  |  |  |  |  | 6.40% |  |  |  |  |  |  |  |  |
NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  | $ 25,300,000 |  |  |  |  |
Incremental increase to current rates |  |  |  |  |  |  |  |  |  | $ 20,000,000 |  |  |  |  |
NDPSC-Renewable Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  | $ 15,400,000 |  |  |  |  |  |
NDPSC-Electric Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  | 9,900,000 |  |  |  |  |  |
Incremental increase to current rates |  |  |  |  |  |  |  |  | $ 4,600,000 |  |  |  |  |  |
NDPSC-Transmission Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost adjustment |  |  |  |  |  |  | $ 6,800,000 |  |  |  |  |  |  |  |
WUTC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  | $ 10,500,000 |  |  |  |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  | 4.20% |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | MTPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement | $ 3,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement increase subsequent year | $ 4,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | SDPUC-Electric [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  | $ 2,700,000 |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | SDPUC-Natural Gas [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  | $ 1,500,000 |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | NDPSC-Renewable Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  | $ 15,100,000 |  |  |  |  |  |  |  |  |  |  |  |
Return on equity, percentage |  |  | 10.50% |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Member] | NDPSC-Electric Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement |  | $ 9,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Incremental increase to current rates |  | $ 4,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Return on equity, percentage |  | 10.50% |  |  |  |  |  |  |  |  |  |  |  |  |

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