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Regulatory matters and revenues subject to refund (Details) - USD ($) | Jun. 30, 2015 | Jun. 25, 2015 | Jun. 24, 2015 | Apr. 16, 2015 | Apr. 10, 2015 | Mar. 31, 2015 | Mar. 18, 2015 | Mar. 11, 2015 | Feb. 06, 2015 | Feb. 03, 2015 | Oct. 03, 2014 | Aug. 11, 2014
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MTPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  | $ 11,800,000 |  |  |  |  |  |  |  |  |  | $ 3,000,000
Percent above current rates requested |  | 21.10% |  |  |  |  |  |  |  |  |  | 3.60%
Amount of requested interim increase in annual rates |  | $ 11,000,000 |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  | $ 2,000,000 |  |
Interim rate increase (decrease), percent |  |  |  |  |  |  |  |  |  | 2.30% |  |
Settlement stipulation |  |  |  |  |  |  | $ 2,500,000 |  |  |  |  |
Settlement stipulation percent |  |  |  |  |  |  | 3.00% |  |  |  |  |
SDPUC-Electric [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested | $ 2,700,000 |  |  |  |  |  |  |  |  |  |  |
Percent above current rates requested | 19.20% |  |  |  |  |  |  |  |  |  |  |
SDPUC-Natural Gas [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested | $ 1,500,000 |  |  |  |  |  |  |  |  |  |  |
Percent above current rates requested | 3.10% |  |  |  |  |  |  |  |  |  |  |
WYPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  | $ 788,000 |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  | 4.10% |
Settlement stipulation |  |  |  | $ 501,000 |  |  |  |  |  |  |  |
Settlement stipulation percent |  |  |  | 2.60% |  |  |  |  |  |  |  |
NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  | $ 4,300,000 |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  | 3.40% |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  | $ 4,300,000 |  |  |  |
Interim rate increase (decrease), percent |  |  |  |  |  |  |  | 3.40% |  |  |  |
OPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  | $ 3,600,000 |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  | 5.10% |  |  |  |  |  |
NDPSC-Environmental [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  | $ 8,100,000 |  |  |  |  |  |  |
WUTC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  | $ 3,900,000 |  |  |  |  |  |  |  |  |
Percent above current rates requested |  |  | 1.60% |  |  |  |  |  |  |  |  |

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