Regulatory matters (Details) - USD ($)	Feb. 08, 2016	Jan. 25, 2016	Jan. 05, 2016	Jan. 01, 2016	Dec. 01, 2015	Nov. 30, 2015	Nov. 25, 2015	Nov. 02, 2015	Oct. 26, 2015	Oct. 21, 2015	Oct. 05, 2015	Sep. 30, 2015	Jun. 30, 2015	Jun. 25, 2015	Mar. 31, 2015
OPUC [Member]
Regulatory matters [Line Items]
Rate increase requested															$ 3,600,000
Percent above current rates requested															5.10%
Settlement agreement								$ 600,000
Settlement agreement, percentage								0.80%
MTPSC [Member]
Regulatory matters [Line Items]
Rate increase requested														$ 11,800,000
Percent above current rates requested														21.10%
Amount of requested interim increase in annual rates														$ 11,000,000
SDPUC-Electric [Member]
Regulatory matters [Line Items]
Rate increase requested													$ 2,700,000
Percent above current rates requested													19.20%
SDPUC-Natural Gas [Member]
Regulatory matters [Line Items]
Rate increase requested													$ 1,500,000
Percent above current rates requested													3.10%
MISO [Member]
Regulatory matters [Line Items]
Revenue requirement											$ 3,800,000
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
Interim rate increase (decrease), percentage						6.40%
NDPSC [Member]
Regulatory matters [Line Items]
Rate increase requested										$ 25,300,000
Incremental increase to current rates										$ 20,000,000
NDPSC-Renewable Rider [Member]
Regulatory matters [Line Items]
Rate increase requested									$ 15,400,000
NDPSC-Electric Rider [Member]
Regulatory matters [Line Items]
Rate increase requested									9,900,000
Incremental increase to current rates									$ 4,600,000
NDPSC-Transmission Adjustment [Member]
Regulatory matters [Line Items]
Cost adjustment							$ 6,800,000
WUTC [Member]
Regulatory matters [Line Items]
Rate increase requested					$ 10,500,000
Percent above current rates requested					4.20%
Subsequent Event [Member] | MTPSC [Member]
Regulatory matters [Line Items]
Settlement agreement	$ 3,000,000
Settlement agreement increase subsequent year	$ 4,400,000
Subsequent Event [Member] | SDPUC-Electric [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount				$ 2,700,000
Subsequent Event [Member] | SDPUC-Natural Gas [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount				$ 1,500,000
Subsequent Event [Member] | NDPSC-Renewable Rider [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount			$ 15,100,000
Return on equity, percentage			10.50%
Subsequent Event [Member] | NDPSC-Electric Rider [Member]
Regulatory matters [Line Items]
Settlement agreement		$ 9,700,000
Incremental increase to current rates		$ 4,400,000
Return on equity, percentage		10.50%