Regulatory matters (Details) - USD ($) $ in Millions	Jul. 07, 2016	Jun. 15, 2016	Jun. 10, 2016	Jun. 01, 2016	Apr. 29, 2016	Feb. 08, 2016	Jan. 25, 2016	Jan. 05, 2016	Jan. 01, 2016	Dec. 01, 2015	Nov. 30, 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
OPUC [Member]
Regulatory matters [Line Items]
Rate increase requested					$ 1.9
Percent above current rates requested					2.80%
WUTC-Pipeline Replacement [Member]
Regulatory matters [Line Items]
Percent of additional revenue requested				2.00%
Replacement cost recovery mechanism				$ 4.6
Incremental pipeline recovery investment				2.4
Incremental operation and maintenance costs				$ 2.2
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
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
Settlement stipulation		$ 1.4
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
Settlement stipulation		$ 1.2
MNPUC [Member]
Regulatory matters [Line Items]
Rate increase requested															$ 1.6
Percent above current rates requested															6.40%
Interim rate increase (decrease), amount											$ 1.5
Interim rate increase (decrease), percent											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
Interim rate increase (decrease), amount							$ 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
WYPSC [Member]
Regulatory matters [Line Items]
Rate increase requested			$ 3.2
Percent above current rates requested			13.10%
Subsequent event [Member] | WUTC [Member]
Regulatory matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 4.0