Regulatory matters and revenues subject to refund (Details) (USD $)	0 Months Ended
	Mar. 18, 2015	Feb. 03, 2015	Aug. 11, 2014	Oct. 03, 2014	Jan. 07, 2015	Mar. 11, 2015	Feb. 06, 2015	Mar. 31, 2015	Apr. 16, 2015	Apr. 10, 2015
MTPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested			$ 3,000,000
Percent above current rates requested			3.60%
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%
WYPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested				788,000
Percent above current rates requested				4.10%
NDPSC-Generation [Member]
Regulatory matters and revenues subject to refund [Line Items]
Annual amount recovered					5,300,000
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%
Subsequent event [Member] | WYPSC [Member]
Regulatory matters and revenues subject to refund [Line Items]
Settlement stipulation									501,000
Settlement stipulation percent									2.60%
Subsequent event [Member] | NDPSC-Environmental [Member]
Regulatory matters and revenues subject to refund [Line Items]
Rate increase requested										$ 8,100,000