Regulatory matters and revenues subject to refund (Details) - USD ($) $ in Millions	Nov. 02, 2015	Oct. 21, 2015	Oct. 05, 2015	Sep. 30, 2015	Aug. 26, 2015	Jun. 30, 2015	Jun. 25, 2015	Jun. 24, 2015	Apr. 10, 2015	Mar. 31, 2015	Mar. 11, 2015	Feb. 06, 2015
NDPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested												$ 4.3
Percent above current rates requested												3.40%
Interim rate increase (decrease), amount											$ 4.3
Interim rate increase (decrease), percent											3.40%
Settlement agreement					$ 2.6
Settlement agreement, percentage					2.00%
OPUC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested										$ 3.6
Percent above current rates requested										5.10%
NDPSC-Environmental [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested									$ 8.1
WUTC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested								$ 3.9
Percent above current rates requested								1.60%
MTPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested							$ 11.8
Percent above current rates requested							21.10%
Amount of requested interim increase in annual rates							$ 11.0
SDPUC-Electric [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested						$ 2.7
Percent above current rates requested						19.20%
SDPUC-Natural Gas [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested						$ 1.5
Percent above current rates requested						3.10%
MNPUC [Member]
Regulatory matters and revenues subject to refund [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%
Subsequent event [Member] | NDPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested		$ 25.3
Incremental increase to current rates		$ 20.0
Subsequent event [Member] | OPUC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Settlement agreement	$ 0.6
Settlement agreement, percentage	0.80%
Subsequent event [Member] | MISO [Member]
Regulatory matters and revenues subject to refund [Line Items]
Revenue requirement			$ 3.8