Fair Value Measurements - Level 3 (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Commodity derivative [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 81	$ 44	$ 60	$ 47
Changes included in earnings	7	9	19	8
Changes in fair value recognized in other comprehensive income	(1)	(2)	(3)	(2)
Changes in fair value recognized in net regulatory assets	(3)	(1)	(5)	(12)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	1	1	1
Fair value, level 3, asset, sales		0		0
Settlements	2	5	14	14
Ending balance	86	56	86	56
Interest Rate Lock Commitments [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	8	14	6	4
Changes included in earnings	34	49	100	103
Changes in fair value recognized in other comprehensive income	0	0	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	8	0	6	0
Fair value, level 3, asset, sales		0		0
Settlements	(37)	(52)	(99)	(96)
Ending balance	13	11	13	11
Auction rate securities [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	18	0	44
Changes included in earnings	0	0	0	0
Changes in fair value recognized in other comprehensive income	0	0	0	6
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
Fair value, level 3, asset, sales		0		(32)
Settlements	0	0	0	0
Ending balance	$ 0	$ 18	$ 0	$ 18