Regulatory matters (Details) - USD ($)	Jul. 31, 2017	Jul. 26, 2017	Jul. 21, 2017	Jun. 30, 2017	Jun. 16, 2017	May 31, 2017	May 30, 2017	Apr. 28, 2017	Apr. 06, 2017	Jan. 17, 2017	Dec. 21, 2016	Dec. 02, 2016	Nov. 21, 2016	Oct. 14, 2016	Aug. 12, 2016	Jun. 10, 2016	Mar. 25, 2016	Mar. 09, 2016	Feb. 10, 2016	Jan. 05, 2016
WYPSC [Member]
Regulatory matters [Line Items]
Rate increase requested																$ 3,200,000
Percent above current rates requested																13.10%
Rate increase approved									$ 2,700,000
Percent above current rates approved									11.10%
IPUC [Member]
Regulatory matters [Line Items]
Rate increase requested															$ 10,200,000
Percent above current rates requested															4.10%
Rate increase approved								$ 4,100,000
Percent above current rates approved								1.60%
Public utilities, requested rate increase (decrease), amended, amount										$ 9,400,000
Public utilities, approved return on equity, percentage								9.50%
MTPSC [Member]
Regulatory matters [Line Items]
Public utilities gas and electric tax tracking adjustment												$ 814,000
Public utilities gas tax tracking adjustment, approved										474,000
Public utilities electric tax tracking adjustment, approved							$ 88,000			$ 251,000
MNPUC-Gas Cost Tariff [Member]
Regulatory matters [Line Items]
Public utilities, gas utility infrastructure cost tariff, requested											$ 456,000
MTPSC-Phase 2 [Member]
Regulatory matters [Line Items]
Rate increase approved																	$ 4,700,000
WUTC-Pipeline Replacement [Member]
Regulatory matters [Line Items]
Public utilities, replacement cost recovery mechanism						$ 1,600,000
Public utilities, replacement cost recovery mechanism, percentage						0.75%
NDPSC-ADP [Member]
Regulatory matters [Line Items]
Estimated project cost				$ 85,000,000
NDPSC [Member]
Regulatory matters [Line Items]
Rate increase requested														$ 13,400,000
Percent above current rates requested														6.60%
Rate increase approved					$ 7,500,000
Percent above current rates approved					3.70%
Interim rate increase (decrease), amount													$ 11,700,000	$ 13,000,000
Interim rate increase (decrease), percent													5.80%	6.50%
NDPSC-Renewable Rider [Member]
Regulatory matters [Line Items]
Public utilities, approved return on equity, percentage					9.65%
NDPSC-Electric Rider [Member]
Regulatory matters [Line Items]
Public utilities, approved return on equity, percentage					9.45%
NDPSC-Riders [Member]
Regulatory matters [Line Items]
Public utilities, approved return on equity, percentage					9.45%
NDPSC-Riders [Member] | NDPSC-Renewable Rider [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount																				$ 15,100,000
Return on equity, percentage																				10.50%
NDPSC-Riders [Member] | NDPSC-Electric Rider [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount																		$ 9,700,000
Return on equity, percentage																		10.50%
NDPSC-Riders [Member] | NDPSC-Transmission Adjustment [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount																			$ 6,800,000
Return on equity, percentage																			10.50%
Subsequent Event [Member] | NDPSC-Natural Gas [Member]
Regulatory matters [Line Items]
Rate increase requested			$ 5,900,000
Percent above current rates requested			5.40%
Interim rate increase (decrease), amount			$ 4,600,000
Interim rate increase (decrease), percent			4.20%
Subsequent Event [Member] | WUTC [Member]
Regulatory matters [Line Items]
Rate increase requested	$ 5,900,000
Percent above current rates requested	2.70%
Subsequent Event [Member] | NDPSC [Member]
Regulatory matters [Line Items]
Public utilities refund from current rates, percentage		19.00%