Derivative Instruments and Hedging Activities - Volumetric Underlying Derivative Transactions (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) MMBTU MWh	Dec. 31, 2020 USD ($) MMBTU MWh
Natural Gas \| Long
Derivatives, Fair Value [Line Items]
Derivative nonmonetary notional amount (energy measure) \| MMBTU	2	1
Power \| Short
Derivatives, Fair Value [Line Items]
Derivative nonmonetary notional amount (energy measure) \| MWh	12	8
Interest \| Long
Derivatives, Fair Value [Line Items]
Derivative, notional amount \| $	$ 1,389	$ 1,600

Nominal or face amount used to calculate payment on derivative.

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=121590274&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=121590274&loc=SL5579245-113959

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant