Derivative Instruments (Details) - Derivative Instruments on the Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Designated as cash flow or fair value hedges
Income tax	$ 0	$ 0	$ 0	$ 1
Total designated as cash flow or fair value hedges, net of tax	(1)	0	(5)	5
Not designated as hedges
Income tax	(10)	2	(1)	2
Total not designated as hedges, net of tax	15	(5)	2	(4)
Total gains (losses) on derivative instruments, net of tax	14	(5)	(3)	1
Cash Flow Hedging [Member]
Designated as cash flow or fair value hedges
Income tax	0	0	0	(1)
Total designated as cash flow or fair value hedges, net of tax	(1)	0	(5)	5
Cost of Sales [Member]
Not designated as hedges
Natural gas contracts - net value adjustments	(3)	(1)	(4)	0
Cost of Sales [Member] | Cash Flow Hedging [Member]
Designated as cash flow or fair value hedges
Natural gas contracts - gain reclassified from OCI	(2)	(1)	(6)	4
Operation and Maintenance Expense [Member] | Cash Flow Hedging [Member]
Designated as cash flow or fair value hedges
Natural gas contracts - gain reclassified from OCI	0	1	(1)	2
Interest Expense [Member] | Cash Flow Hedging [Member]
Designated as cash flow or fair value hedges
Natural gas contracts - gain reclassified from OCI	1	0	2	0
Operating Revenues [Member]
Not designated as hedges
Natural gas contracts - net value adjustments	$ 28	$ (6)	$ 7	$ (6)