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Regulatory matters (Details) - USD ($)	Apr. 28, 2017	Apr. 07, 2017	Apr. 06, 2017	Jan. 17, 2017	Jan. 10, 2017	Dec. 21, 2016	Dec. 12, 2016	Dec. 02, 2016	Nov. 21, 2016	Oct. 14, 2016	Sep. 06, 2016	Aug. 12, 2016	Jun. 10, 2016	Apr. 29, 2016	Mar. 25, 2016	Mar. 09, 2016	Feb. 10, 2016	Jan. 05, 2016	Jan. 01, 2016	Sep. 30, 2015
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), percent																			6.40%
Rate increase approved											$ 1,100,000
Percent above current rates approved											5.20%
OPUC [Member]
Regulatory matters [Line Items]
Rate increase requested														$ 1,900,000
Percent above current rates requested														2.80%
Rate increase approved							$ 754,000
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%
Public utilities, requested rate increase (decrease), amended, amount				$ 9,400,000
MISO [Member]
Regulatory matters [Line Items]
Revenue requirement					$ 11,100,000
MTPSC [Member]
Regulatory matters [Line Items]
Public utilities gas and electric tax tracking adjustment								$ 814,000
Public utilities gas tax tracking adjustment, approved				474,000
Public utilities electric tax tracking adjustment, approved				$ 251,000
MNPUC-Gas Cost Tariff [Member]
Regulatory matters [Line Items]
Public utilities, gas utility infrastructure cost tariff, requested						$ 456,000
MTPSC-Phase 2 [Member]
Regulatory matters [Line Items]
Rate increase approved															$ 4,700,000
NDPSC [Member]
Regulatory matters [Line Items]
Rate increase requested										$ 13,400,000
Percent above current rates requested										6.60%
Interim rate increase (decrease), amount									$ 11,700,000	$ 13,000,000
Interim rate increase (decrease), percent									5.80%	6.50%
NDPSC-Riders [Member] | NDPSC-Transmission Adjustment [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount																	$ 6,800,000
Return on equity, percentage																	10.50%
NDPSC-Riders [Member] | NDPSC-Renewable Rider [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount																		$ 15,100,000
Return on equity, percentage																		10.50%
NDPSC-Riders [Member] | NDPSC-Electric Rider [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount																$ 9,700,000
Return on equity, percentage																10.50%
Subsequent event [Member] | WYPSC [Member]
Regulatory matters [Line Items]
Rate increase approved			$ 2,700,000
Percent above current rates approved			11.10%
Subsequent event [Member] | IPUC [Member]
Regulatory matters [Line Items]
Rate increase approved	$ 4,100,000
Percent above current rates approved	1.60%
Public utilities, approved return on equity, percentage	9.50%
Subsequent event [Member] | NDPSC [Member]
Regulatory matters [Line Items]
Settlement agreement		$ 7,500,000
Settlement agreement, percentage		3.70%
Public utilities refund from current rates, percentage		19.00%
Subsequent event [Member] | NDPSC-Renewable Rider [Member]
Regulatory matters [Line Items]
Settlement agreement return on equity, percentage		9.65%
Subsequent event [Member] | NDPSC-Electric Rider [Member]
Regulatory matters [Line Items]
Settlement agreement return on equity, percentage		9.45%
Subsequent event [Member] | NDPSC-Riders [Member]
Regulatory matters [Line Items]
Settlement agreement return on equity, percentage		9.45%

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