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Accounting for Derivative Instruments and Hedging Activities - Net notional volume buy/sell of open derivative transactions (Details) shares in Millions, mw_per_day in Millions, bbl in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) MWh MMBTU mw_per_day T shares bbl	Dec. 31, 2018 USD ($) MWh MMBTU mw_per_day T shares bbl	Sep. 30, 2018 USD ($)
Volumetric Underlying Derivative Transactions
Balance, tax | $			$ 0
Short | Emissions
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass (ton) | T	2	2
Short | Natural Gas
Volumetric Underlying Derivative Transactions
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	(273)	(330)
Short | Capacity
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, flow rate (in MW/Day) | mw_per_day	(1)	(1)
Long | Renewable Energy Certificates
Volumetric Underlying Derivative Transactions
Derivative, non-monetary notional amount (in shares) | shares	1	1
Long | Coal
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass (ton) | T	4	13
Long | Oil
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount (in barrels) | bbl	0	1
Long | Power
Volumetric Underlying Derivative Transactions
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	(37)	(1)
Long | Interest
Volumetric Underlying Derivative Transactions
Derivative notional amount (in usd) | $	$ 0	$ 1,000

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Volumetric Underlying Derivative Transactions [Abstract]

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- Definition

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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

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