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Regulatory matters (Details) - USD ($)	Oct. 27, 2016	Oct. 14, 2016	Oct. 06, 2016	Sep. 06, 2016	Aug. 12, 2016	Jul. 07, 2016	Jun. 15, 2016	Jun. 10, 2016	Jun. 01, 2016	Apr. 29, 2016	Mar. 09, 2016	Feb. 10, 2016	Jan. 05, 2016	Jan. 01, 2016	Dec. 01, 2015	Sep. 30, 2015	Jun. 30, 2015
SDPUC-Electric [Member]
Regulatory matters [Line Items]
Rate increase requested																	$ 2,700,000
Percent above current rates requested																	19.20%
Interim rate increase (decrease), amount														$ 2,700,000
Rate increase approved							$ 1,400,000
SDPUC-Natural Gas [Member]
Regulatory matters [Line Items]
Rate increase requested																	$ 1,500,000
Percent above current rates requested																	3.10%
Interim rate increase (decrease), amount														1,500,000
Rate increase approved							$ 1,200,000
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
Rate increase approved				$ 1,100,000
Interim rate increase (decrease), percent														6.40%
Percent above current rates approved				5.20%
NDPSC-Renewable Rider [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount													$ 15,100,000
Return on equity, percentage													10.50%
NDPSC-Electric Rider [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount											$ 9,700,000
Return on equity, percentage											10.50%
NDPSC-Transmission Adjustment [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount												$ 6,800,000
Return on equity, percentage												10.50%
WUTC [Member]
Regulatory matters [Line Items]
Rate increase requested															$ 10,500,000
Percent above current rates requested															4.20%
Rate increase approved						$ 4,000,000
OPUC [Member]
Regulatory matters [Line Items]
Rate increase requested										$ 1,900,000
Percent above current rates requested										2.80%
WUTC-Pipeline Replacement [Member]
Regulatory matters [Line Items]
Replacement cost recovery mechanism									$ 4,600,000
Percent of additional revenue requested									2.00%
Incremental pipeline recovery investment									$ 2,400,000
Incremental operation and maintenance costs									$ 2,200,000
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%
Subsequent event [Member] | OPUC [Member]
Regulatory matters [Line Items]
Settlement agreement			$ 754,000
Subsequent event [Member] | WUTC-Pipeline Replacement [Member]
Regulatory matters [Line Items]
Incremental pipeline recovery investment, allowed amount	$ 1,900,000
Subsequent event [Member] | NDPSC [Member]
Regulatory matters [Line Items]
Rate increase requested		$ 13,400,000
Percent above current rates requested		6.60%
Public utilities, interim rate increase (decrease), amount requested		$ 13,000,000

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