Derivative and Hedging Activities - Quantitative Disclosures Level 3 Table (Details) - Energy Related Derivative [Member] $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($)
Oil [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average	27.29%
Fair value, measurement with unobservable inputs reconciliations, recurring basis, liability value	$ (31)
Oil [Member] \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average	19.20%
Oil [Member] \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average	30.57%
Natural gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average	20.73%
Fair value, measurement with unobservable inputs reconciliations, recurring basis, asset value	$ 7
Natural gas [Member] \| Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average	14.74%
Natural gas [Member] \| Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted Average	40.22%

Weighted average expected volatility of stock price.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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