Fair Value Measurements - Level 3 Rollforward Derivatives (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Net derivative balance at beginning of period	$ (43)	$ (41)	$ (41)	$ (49)
Deferred as a regulatory asset or liability	(6)	(3)	(11)	4
Settlements	8	6	12	10
Transfers into Level 3 from Level 2	(1)	0	(5)	(2)
Transfers from Level 3 into Level 2	1	(1)	4	(2)
Net derivative balance at end of period	(41)	(39)	(41)	(39)
Net unrealized gains included in earnings related to instruments still held at end of period	$ 0	$ 0	$ 0	$ 0