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
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%