Fair Value Measurements - Level 3 (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Commodity derivative [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 34	$ 9	$ 51	$ 60
Changes included in earnings	6	17	17	(4)
Changes in fair value recognized in other comprehensive income	(2)	0	(5)	4
Changes in fair value recognized in net regulatory assets	(4)	(3)	(21)	(3)
Purchases	0		1
Settlements	9	(2)	0	(1)
Transfers from Level 2	0	0	0	(35)
Ending balance	43	21	43	21
Interest Rate Lock Commitments [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	5	0	0	0
Changes included in earnings	25	0	70	0
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
Purchases	0		0
Settlements	(23)	0	(66)	0
Transfers from Level 2	0	0	3	0
Ending balance	7	0	7	0
Auction rate securities [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	45	46	45	44
Changes included in earnings	0	0	0	0
Changes in fair value recognized in other comprehensive income	(1)	(1)	(1)	1
Changes in fair value recognized in net regulatory assets	0	0	0	0
Purchases	0		0
Settlements	0	0	0	0
Transfers from Level 2	0	0	0	0
Ending balance	$ 44	$ 45	$ 44	$ 45