Fair Value Measurements - Level 3 Rollforward Derivatives (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Net derivative beginning balance	$ (41,000,000)	$ (49,000,000)
Deferred as a regulatory asset or liability	(11,000,000)	4,000,000
Settlements	0	0
Transfers into Level 3 from Level 2	(1,000,000)	(3,000,000)
Transfers from Level 3 into Level 2	4,000,000	(1,000,000)
Net derivative ending balance	(49,000,000)	(49,000,000)
Net unrealized gains included in earnings related to instruments still held at end of period	$ 0	$ 0