Commitments and Contingencies Environmental (Details) $ in Millions	3 Months Ended
Mar. 31, 2017 USD ($)	Jul. 01, 2018
Colstrip Unit 4 [Member]
Jointly owned utility plant ownership percentage	30.00%
Environmental obligation, estimated capital Expenditures	$ 90.0
Coyote Generating Facility [Member]
Jointly owned utility plant ownership percentage	10.00%
Environmental remediation obligations [Member]
Environmental remediation obligation, minimum	$ 27.9
Environmental remediation obligation, maximum	32.6
Accrual for environmental loss contingencies	31.2
Combined Manufacturing Sites [Member] \| Manufactured Gas Plants [Member]
Accrual for environmental loss contingencies	24.5
Aberdeen South Dakota Site [Member] \| Manufactured Gas Plants [Member]
Accrual for environmental loss contingencies	10.5
Environmental remediation obligation next 5 years	$ 6.0
Number of years for environmental remediation obligation to be incurred	5 years
Scenario, Forecast [Member] \| Coyote Generating Facility [Member]
NOx emissions per million Btu as calculated on a 30 day rolling average basis		0.5

Estimated capital expenditure projections related to environmental obligations.

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- Definition

Environmental Remediation Obligation Maximum

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- Definition

Environmental Remediation Obligation Minimum

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- Definition

Environmental Remediation Obligation, To be Incurred During Next 5 years

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- Definition

NOx Emissions Per Million Btu As Calculated On A 30-day Rolling Basis

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- Definition

Number Of Years For Environmental Remediation Obligation To Be Incurred

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- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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=6500719&loc=d3e659983-123033

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