Fair Value Measurements - Level 3 (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Commodity derivative [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 52	$ 86	$ 97	$ 99
Changes included in earnings	(1)	8	(4)	5
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)		1		1
Changes in fair value recognized in net regulatory assets	(16)	(12)	(56)	(23)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	1	3	3	4
Settlements	8	2	4	2
Ending balance	44	86	44	86
Interest Rate Lock Commitments [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	45	18	14	10
Changes included in earnings	264	94	336	147
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)		0		0
Changes in fair value recognized in net regulatory assets	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Settlements	(231)	(89)	(272)	(134)
Ending balance	$ 78	$ 23	$ 78	$ 23