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Regulatory matters (Details) - USD ($) $ in Millions | Apr. 29, 2016 | Feb. 08, 2016 | Jan. 25, 2016 | Jan. 05, 2016 | Jan. 01, 2016 | Dec. 01, 2015 | Nov. 25, 2015 | Oct. 26, 2015 | Oct. 21, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Jun. 25, 2015
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NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  | $ 25.3 |  |  |
Incremental increase to current rates |  |  |  |  |  |  |  |  | $ 20.0 |  |  |
WUTC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  | $ 10.5 |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  | 4.20% |  |  |  |  |  |
MTPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  | $ 11.8
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  | 21.10%
Settlement agreement |  | $ 3.0 |  |  |  |  |  |  |  |  |  |
Settlement agreement increase subsequent year |  | $ 4.4 |  |  |  |  |  |  |  |  |  |
Amount of requested interim increase in annual rates |  |  |  |  |  |  |  |  |  |  |  | $ 11.0
SDPUC-Electric [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  | $ 2.7 |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  | 19.20% |
Interim rate increase (decrease), amount |  |  |  |  | $ 2.7 |  |  |  |  |  |  |
SDPUC-Natural Gas [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  | $ 1.5 |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  | 3.10% |
Interim rate increase (decrease), amount |  |  |  |  | $ 1.5 |  |  |  |  |  |  |
MNPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  | $ 1.6 |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  | 6.40% |  |
Amount of requested interim increase in annual rates |  |  |  |  |  |  |  |  |  | $ 1.5 |  |
Requested percentage of interim rate increase |  |  |  |  |  |  |  |  |  | 6.40% |  |
NDPSC-Renewable Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  | $ 15.4 |  |  |  |
Interim rate increase (decrease), amount |  |  |  | $ 15.1 |  |  |  |  |  |  |  |
Return on equity, percentage |  |  |  | 10.50% |  |  |  |  |  |  |  |
NDPSC-Electric Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  | 9.9 |  |  |  |
Settlement agreement |  |  | $ 9.7 |  |  |  |  |  |  |  |  |
Incremental increase to current rates |  |  | $ 4.4 |  |  |  |  | $ 4.6 |  |  |  |
Return on equity, percentage |  |  | 10.50% |  |  |  |  |  |  |  |  |
NDPSC-Transmission Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Return on equity, percentage |  |  |  |  |  |  | 10.50% |  |  |  |  |
Cost adjustment |  |  |  |  |  |  | $ 6.8 |  |  |  |  |
Subsequent event [Member] | OPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested | $ 1.9 |  |  |  |  |  |  |  |  |  |  |
Percent above current rates requested | 2.80% |  |  |  |  |  |  |  |  |  |  |

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