Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions	3 Months Ended
Mar. 31, 2017 USD ($) MW	Mar. 31, 2016 USD ($)
Square Butte [Member] \| Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Total Debt Outstanding	$ 313.7
Cost of Power Purchased	20.3	$ 18.5
Pro Rata Share of PPA Counterparty Interest Expense	$ 2.3	$ 2.4
Square Butte [Member] \| Square Butte PPA [Member] \| Square Butte Coal-fired Unit [Member]
Power Purchase Agreements [Line Items]
Generating Capacity (MW) \| MW	455
Output Entitlement	50.00%
Minnkota Power [Member] \| Square Butte Coal-fired Unit [Member] \| Minnkota Power PSA [Member]
Power Purchase Agreements [Line Items]
Output Entitlement	28.00%	28.00%

Capacity of a generating facility, unit, or project in megawatts.

No definition available.

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- Definition

Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.D)
-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

The allocable portion of interest included in charges under a long-term contract to purchase electric power.

+ References

+ Details

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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

No definition available.

+ Details

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- Definition

Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ References

+ Details

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- Definition

Amount of operating expense for purchased power procured under arrangements extending beyond one year or the normal operating cycle, if longer, by regulated operation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(b))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 10.D)
-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

+ Details

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- Details

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- Details

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- Details

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- Details

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- Details

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