Regulatory matters and revenues subject to refund (Details) - USD ($)	Jun. 30, 2015	Jun. 25, 2015	Jun. 24, 2015	Apr. 16, 2015	Apr. 10, 2015	Mar. 31, 2015	Mar. 18, 2015	Mar. 11, 2015	Feb. 06, 2015	Feb. 03, 2015	Oct. 03, 2014	Aug. 11, 2014
MTPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested		$ 11,800,000										$ 3,000,000
Percent above current rates requested		21.10%										3.60%
Amount of requested interim increase in annual rates		$ 11,000,000
Interim rate increase (decrease), amount										$ 2,000,000
Interim rate increase (decrease), percent										2.30%
Settlement stipulation							$ 2,500,000
Settlement stipulation percent							3.00%
SDPUC-Electric [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested	$ 2,700,000
Percent above current rates requested	19.20%
SDPUC-Natural Gas [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested	$ 1,500,000
Percent above current rates requested	3.10%
WYPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested											$ 788,000
Percent above current rates requested											4.10%
Settlement stipulation				$ 501,000
Settlement stipulation percent				2.60%
NDPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested									$ 4,300,000
Percent above current rates requested									3.40%
Interim rate increase (decrease), amount								$ 4,300,000
Interim rate increase (decrease), percent								3.40%
OPUC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested						$ 3,600,000
Percent above current rates requested						5.10%
NDPSC-Environmental [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested					$ 8,100,000
WUTC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested			$ 3,900,000
Percent above current rates requested			1.60%