Regulatory Assets and Liabilities - Amount of regulatory assets and liabilities recorded on consolidated balance sheet (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2016	Dec. 31, 2015
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current		$ 19,958	$ 18,904
Regulatory Liability - Current		5,934	3,322
Net Regulatory Assets Position - Current		14,024	15,582
Regulatory Assets - Long-Term		118,123	127,707
Regulatory Liabilities - Long-Term		79,835	77,432
Net Regulatory Asset Position - Long-Term		38,288	50,275
Regulatory Assets - Total		138,081	146,611
Regulatory Liabilities - Total		85,769	80,754
Net Regulatory Asset Position - Total		52,312	65,857
Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[1]	7,439	7,439
Regulatory Assets - Long-Term	[1]	94,671	99,293
Regulatory Assets - Total	[1]	$ 102,110	$ 106,732
Regulatory Assets - Remaining Recovery/Refund Period	[1]	see below	see below
Deferred Marked-to-Market Loss
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[1]	$ 4,063	$ 4,063
Regulatory Assets - Long-Term	[1]	7,483	10,530
Regulatory Assets - Total	[1]	$ 11,546	$ 14,593
Regulatory Assets - Remaining Recovery/Refund Period	[1]	51 months	60 months
Conservation Improvement Program Costs and Incentives
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[2]	$ 4,286	$ 4,411
Regulatory Assets - Long-Term	[2]	3,079	4,266
Regulatory Assets - Total	[2]	$ 7,365	$ 8,677
Regulatory Assets - Remaining Recovery/Refund Period	[2]	24 months	18 months
Accumulated ARO Accretion/Depreciation Adjustment
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[1]
Regulatory Assets - Long-Term	[1]	6,031	5,672
Regulatory Assets - Total	[1]	$ 6,031	$ 5,672
Regulatory Assets - Remaining Recovery/Refund Period	[1]	asset lives	asset lives
Big Stone II Unrecovered Project Costs - Minnesota
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[1]	$ 619	$ 942
Regulatory Assets - Long-Term	[1]	2,444	2,620
Regulatory Assets - Total	[1]	$ 3,063	$ 3,562
Regulatory Assets - Remaining Recovery/Refund Period	[1]	55 months	84 months
Minnesota Transmission Cost Recovery Rider Accrued Revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[2]		$ 576
Regulatory Assets - Long-Term	[2]
Regulatory Assets - Total	[2]		$ 576
Regulatory Assets - Remaining Recovery/Refund Period	[2]		12 months
North Dakota Renewable Resource Rider Accrued Revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[2]	$ 1,608
Regulatory Assets - Long-Term	[2]	826	1,266
Regulatory Assets - Total	[2]	$ 2,434	$ 1,266
Regulatory Assets - Remaining Recovery/Refund Period	[2]	18 months	15 months
Debt Reacquisition Premiums
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[1]	$ 349	$ 351
Regulatory Assets - Long-Term	[1]	1,278	1,539
Regulatory Assets - Total	[1]	$ 1,627	$ 1,890
Regulatory Assets - Remaining Recovery/Refund Period	[1]	192 months	201 months
Deferred Income Taxes
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[1]
Regulatory Liability - Current
Regulatory Assets - Long-Term	[1]	1,157	1,455
Regulatory Liabilities - Long-Term		918	1,110
Regulatory Assets - Total	[1]	1,157	1,455
Regulatory Liabilities - Total		$ 918	$ 1,110
Regulatory Assets - Remaining Recovery/Refund Period	[1]	asset lives	asset lives
Regulatory Liabilities - Remaining Recovery/Refund Period		asset lives	asset lives
MISO Schedule 26/26A Transmission Cost Recovery Rider True-up
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[2]	$ 474	$ 698
Regulatory Liability - Current
Regulatory Assets - Long-Term	[2]	43	355
Regulatory Liabilities - Long-Term		80
Regulatory Assets - Total	[2]	517	$ 1,053
Regulatory Liabilities - Total		$ 80
Regulatory Assets - Remaining Recovery/Refund Period	[2]	27 months	24 months
Regulatory Liabilities - Remaining Recovery/Refund Period		27 months
Big Stone II Unrecovered Project Costs - South Dakota
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[2]	$ 101	$ 100
Regulatory Assets - Long-Term	[2]	567	643
Regulatory Assets - Total	[2]	$ 668	$ 743
Regulatory Assets - Remaining Recovery/Refund Period	[2]	80 months	89 months
North Dakota Transmission Cost Recovery Rider Accrued Revenues [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[2]
Regulatory Assets - Long-Term	[2]	544
Regulatory Assets - Total	[2]	$ 544
Regulatory Assets - Remaining Recovery/Refund Period	[2]	27 months
Minnesota Environmental Cost Recovery Rider Accrued Refund
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current		$ 370	$ 777
Regulatory Liabilities - Long-Term
Regulatory Liabilities - Total		$ 370	$ 777
Regulatory Liabilities - Remaining Recovery/Refund Period		12 months	12 months
Minnesota Renewable Resource Rider Accrued Revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[2]	$ 46
Regulatory Assets - Long-Term	[2]		68
Regulatory Assets - Total	[2]	$ 46	$ 68
Regulatory Assets - Remaining Recovery/Refund Period	[2]		see below
Regulatory Assets - Remaining Recovery/Refund Period	[2]	12 months
Accumulated Reserve for Estimated Removal Costs - Net of Salvage
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current
Regulatory Liabilities - Long-Term		77,603	74,948
Regulatory Liabilities - Total		$ 77,603	$ 74,948
Regulatory Liabilities - Remaining Recovery/Refund Period		asset lives	asset lives
North Dakota Renewable Resource Rider Accrued Refund
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current			$ 68
Regulatory Liabilities - Long-Term
Regulatory Liabilities - Total			$ 68
Regulatory Liabilities - Remaining Recovery/Refund Period			12 months
Revenue for Rate Case expenses Subject to Refund - Minnesota
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current		$ 712
Regulatory Liabilities - Long-Term		385	1,279
Regulatory Liabilities - Total		$ 1,097	$ 1,279
Regulatory Liabilities - Remaining Recovery/Refund Period			see below
Regulatory Liabilities - Remaining Recovery/Refund Period		19 months
Deferred Gain on Sale of Utility Property - Minnesota Portion
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current			$ 5
Regulatory Liabilities - Long-Term			95
Regulatory Liabilities - Total			$ 100
Regulatory Liabilities - Remaining Recovery/Refund Period			216 months
South Dakota Environmental Cost Recovery Rider Accrued Refund
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current		$ 296	$ 185
Regulatory Liabilities - Long-Term
Regulatory Liabilities - Total		$ 296	$ 185
Regulatory Liabilities - Remaining Recovery/Refund Period		12 months	12 months
Minnesota Deferred Rate Case Expenses Subject to Recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[1]	$ 748	$ 291
Regulatory Assets - Long-Term	[1]
Regulatory Assets - Total	[1]	$ 748	$ 291
Regulatory Assets - Remaining Recovery/Refund Period	[1]	12 months	12 months
South Dakota Transmission Cost Recovery Rider Accrued Revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Asset - Current	[2]	$ 225	$ 33
Regulatory Assets - Long-Term	[2]
Regulatory Assets - Total	[2]	$ 225	$ 33
Regulatory Assets - Remaining Recovery/Refund Period	[2]	12 months	12 months
Refundable Fuel Clause Adjustment Revenues
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current		$ 2,301	$ 1,834
Regulatory Liabilities - Long-Term
Regulatory Liabilities - Total		$ 2,301	$ 1,834
Regulatory Liabilities - Remaining Recovery/Refund Period		12 months	12 months
North Dakota Transmission Cost Recovery Rider Accrued Refund
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current		$ 638	$ 132
Regulatory Liabilities - Long-Term		758
Regulatory Liabilities - Total		$ 1,396	$ 132
Regulatory Assets - Remaining Recovery/Refund Period		27 months
Regulatory Liabilities - Remaining Recovery/Refund Period		24 months	12 months
North Dakota Environmental Cost Recovery Rider Accrued Refund
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current		$ 256	$ 321
Regulatory Liabilities - Long-Term
Regulatory Liabilities - Total		$ 256	$ 321
Regulatory Liabilities - Remaining Recovery/Refund Period		12 months	12 months
Minnesota Transmission Cost Recovery Rider Accrued Refund
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current		$ 1,356
Regulatory Liabilities - Long-Term		1,356
Regulatory Liabilities - Total
Regulatory Assets - Remaining Recovery/Refund Period	[2]		12 months
Regulatory Liabilities - Remaining Recovery/Refund Period		12 months
Other
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Liability - Current		$ 5
Regulatory Liabilities - Long-Term		91
Regulatory Liabilities - Total		$ 96
Regulatory Liabilities - Remaining Recovery/Refund Period		207 months

[1] Costs subject to recovery without a rate of return.

[2] Amount eligible for recovery under an alternative revenue program which includes an incentive or rate of return.