Operating Segments - Customers (Details) - Sale of Natural Gas - customer	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Concentration risk
Number of customers	29	28
Customer Concentration risk
Concentration risk
Number of customers	2	2
Threshold concentration risk for reporting (as a percent)	10.00%	10.00%
Customer Concentration risk \| Customer One
Concentration risk
Concentration risk (as a percent)	47.00%	46.00%
Customer Concentration risk \| Customer Two
Concentration risk
Concentration risk (as a percent)	27.00%	21.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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