Regulatory matters (Details) - USD ($)	Jan. 17, 2017	Jan. 10, 2017	Dec. 28, 2016	Dec. 21, 2016	Dec. 12, 2016	Dec. 02, 2016	Nov. 23, 2016	Nov. 21, 2016	Oct. 27, 2016	Oct. 14, 2016	Sep. 06, 2016	Aug. 12, 2016	Jun. 10, 2016	Jun. 01, 2016	Apr. 29, 2016	Mar. 09, 2016	Feb. 10, 2016	Jan. 05, 2016	Jan. 01, 2016	Sep. 30, 2015
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%
Public utilities, approved rate increase (decrease), amount											$ 1,100,000
Public utilities, approved rate increase (decrease), percentage											5.20%
NDPSC-Renewable Rider [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount																		$ 15,100,000
Public utilities, requested return on equity, percentage																		10.50%
NDPSC-Electric Rider [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount																$ 9,700,000
Public utilities, requested return on equity, percentage																10.50%
NDPSC-Transmission Adjustment [Member]
Regulatory matters [Line Items]
Interim rate increase (decrease), amount																	$ 6,800,000
Public utilities, requested return on equity, percentage																	10.50%
OPUC [Member]
Regulatory matters [Line Items]
Rate increase requested															$ 1,900,000
Percent above current rates requested															2.80%
Public utilities, approved rate increase (decrease), amount					$ 754,000
WUTC-Pipeline Replacement [Member]
Regulatory matters [Line Items]
Public utilities, replacement cost recovery mechanism														$ 4,600,000
Public utilities, requested additional revenue, percentage														2.00%
Incremental pipeline replacement investment														$ 2,400,000
Incremental operation and maintenance costs														$ 2,200,000
Incremental pipeline recovery investment, allowed amount									$ 1,900,000
WYPSC [Member]
Regulatory matters [Line Items]
Rate increase requested													$ 3,200,000
Percent above current rates requested													13.10%
Settlement stipulation			$ 2,700,000
Settlement stipulation percent			11.10%
IPUC [Member]
Regulatory matters [Line Items]
Rate increase requested												$ 10,200,000
Percent above current rates requested												4.10%
Public utilities, requested rate increase (decrease), amended, amount							$ 9,600,000
NDPSC [Member]
Regulatory matters [Line Items]
Rate increase requested										$ 13,400,000
Percent above current rates requested										6.60%
Interim rate increase (decrease), amount								$ 11,700,000		$ 13,000,000
Interim rate increase (decrease), percentage								5.80%		6.50%
MTPSC [Member]
Regulatory matters [Line Items]
Public utilities gas and electric tax tracking adjustment						$ 814,000
MNPUC-Gas Cost Tariff [Member]
Regulatory matters [Line Items]
Public utilities, gas utility infrastructure cost tariff, requested				$ 456,000
Subsequent Event [Member] | MISO [Member]
Regulatory matters [Line Items]
Revenue requirement		$ 11,100,000
Subsequent Event [Member] | MTPSC [Member]
Regulatory matters [Line Items]
Public utilities gas tax tracking adjustment, approved	$ 474,000
Public utilities electric tax tracking adjustment, approved	$ 251,000