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Regulatory matters and revenues subject to refund (Details) - USD ($) $ in Millions | Nov. 02, 2015 | Oct. 21, 2015 | Oct. 05, 2015 | Sep. 30, 2015 | Aug. 26, 2015 | Jun. 30, 2015 | Jun. 25, 2015 | Jun. 24, 2015 | Apr. 10, 2015 | Mar. 31, 2015 | Mar. 11, 2015 | Feb. 06, 2015
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NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  | $ 4.3
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  | 3.40%
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  | $ 4.3 |
Interim rate increase (decrease), percent |  |  |  |  |  |  |  |  |  |  | 3.40% |
Settlement agreement |  |  |  |  | $ 2.6 |  |  |  |  |  |  |
Settlement agreement, percentage |  |  |  |  | 2.00% |  |  |  |  |  |  |
OPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  | $ 3.6 |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  | 5.10% |  |
NDPSC-Environmental [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  | $ 8.1 |  |  |
WUTC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  | $ 3.9 |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  | 1.60% |  |  |  |
MTPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  | $ 11.8 |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  | 21.10% |  |  |  |  |
Amount of requested interim increase in annual rates |  |  |  |  |  |  | $ 11.0 |  |  |  |  |
SDPUC-Electric [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  | $ 2.7 |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  | 19.20% |  |  |  |  |  |
SDPUC-Natural Gas [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  | $ 1.5 |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  | 3.10% |  |  |  |  |  |
MNPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [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% |  |  |  |  |  |  |  |
Subsequent event [Member] | NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  | $ 25.3 |  |  |  |  |  |  |  |  |  |
Incremental increase to current rates |  | $ 20.0 |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | OPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement | $ 0.6 |  |  |  |  |  |  |  |  |  |  |
Settlement agreement, percentage | 0.80% |  |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | MISO [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue requirement |  |  | $ 3.8 |  |  |  |  |  |  |  |  |

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