Fair Value Measurements - Level 3 (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Commodity derivative [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 72	$ 58	$ 60	$ 47
Changes included in earnings	0	(20)	12	(1)
Changes in fair value recognized in other comprehensive income	0	6	(2)	0
Changes in fair value recognized in net regulatory assets	(3)	(5)	(2)	(11)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	1		1
Fair value, level 3, asset, sales		0		0
Settlements	11	5	12	9
Ending balance	81	44	81	44
Interest Rate Lock Commitments [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	9	11	6	4
Changes included in earnings	39	29	66	54
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	0		(2)
Fair value, level 3, asset, sales		0		0
Settlements	(40)	0	(62)	(44)
Ending balance	8	14	8	14
Auction rate securities [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	26	0	44
Changes included in earnings	0	0	0	0
Changes in fair value recognized in other comprehensive income	0	2	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
Fair value, level 3, asset, sales		(10)		(32)
Settlements	0	(26)	0	0
Ending balance	$ 0	$ 18	$ 0	$ 18