Fair Value Measurements - Level 3 (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Commodity derivative [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 9	$ 27	$ 60	$ 32
Changes included in earnings	17	12	(4)	16
Changes in fair value recognized in other comprehensive income	0	0	4	(5)
Changes in fair value recognized in net regulatory assets	(3)	(1)	(3)	1
Purchases		0		2
Settlements	(2)	1	(1)	(7)
Transfers from level 2	0		(35)
Ending balance	21	39	21	39
Auction rate securities [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	46	42	44	41
Changes included in earnings	0	0	0	0
Changes in fair value recognized in other comprehensive income	(1)	1	1	2
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
Ending balance	$ 45	$ 43	$ 45	$ 43