Fair Value Measurements - Level 3 Rollforward Derivatives (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (41,000,000)	$ (53,000,000)	$ (49,000,000)	$ (48,000,000)
Total net gains (losses) realized/unrealized: Deferred as a regulatory asset or liability	(3,000,000)	4,000,000	4,000,000	(2,000,000)
Settlements	6,000,000	6,000,000	10,000,000	8,000,000
Transfers into Level 3 from Level 2	0	(1,000,000)	(2,000,000)	(1,000,000)
Transfers from Level 3 into Level 2	(1,000,000)	0	(2,000,000)	(1,000,000)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(39,000,000)	(44,000,000)	(39,000,000)	(44,000,000)
Fair value measurement transfers			$ 0