Jointly owned facilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Jointly owned electricity generation plant \| Big Stone Station
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	22.70%
Utility plant in service	$ 152,836	$ 156,534
Construction work in progress	518	92
Less accumulated depreciation	46,266	49,345
Utility plant in services net	$ 107,088	107,281
Jointly owned electricity generation plant \| Coyote Station
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 160,235	155,236
Construction work in progress	21	1,920
Less accumulated depreciation	107,638	105,565
Utility plant in services net	$ 52,618	51,591
Jointly owned electricity generation plant \| Wygen III
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	25.00%
Utility plant in service	$ 67,869	65,382
Construction work in progress	112	220
Less accumulated depreciation	10,482	9,174
Utility plant in services net	$ 57,499	56,428
Jointly Owned Electricity Transmission and Distribution System \| BSSE
Jointly Owned Utility Plant Interests [Line Items]
Proportionate ownership share	50.00%
Utility plant in service	$ 105,767
Construction work in progress	0	105,846
Less accumulated depreciation	1,232
Utility plant in services net	$ 104,535	$ 105,846

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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- Definition

The total period end amount of plant in service and construction in progress net of accumulated depreciation for the utility's share of jointly owned electricity generation plant.

+ References

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- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

+ References

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- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

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- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

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