Fair Value Measurements - Level 3 Quantitative Information (Details) - Forward Contracts $ in Thousands	Dec. 31, 2019 USD ($) $ / MWh	Dec. 31, 2018 USD ($) $ / MWh
Electricity forward contracts \| Minimum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	22.18	17.88
Electricity forward contracts \| Maximum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	22.18	37.03
Electricity forward contracts \| Weighted-Average
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	22.18	26.10
Natural gas forward contracts \| Minimum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	2.33	1.79
Natural gas forward contracts \| Maximum
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	2.78	2.92
Natural gas forward contracts \| Weighted-Average
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Significant Unobservable Input \| $ / MWh	2.49	2.48
(Level 3)
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets \| $	$ 33	$ 2
Financial and NonFinancial Liabilities, Fair Value Disclosure \| $	3,429	8,216
(Level 3) \| Electricity forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets \| $	33	0
Financial and NonFinancial Liabilities, Fair Value Disclosure \| $	819	2,456
(Level 3) \| Natural gas forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets \| $	0	2
Financial and NonFinancial Liabilities, Fair Value Disclosure \| $	$ 2,610	$ 5,760

Value of input used to measure net derivative asset (liability).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

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+ References

No definition available.

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Fair value of financial and nonfinancial obligations.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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