Derivative Instruments - Derivative Instruments on the Consolidated Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Designated as cash flow or fair value hedges
Income tax	$ (1)	$ 2	$ (1)
Total designated as cash flow or fair value hedges, net of tax	(8)	3	(3)
Not designated as hedges
Income tax	(19)	(54)	34
Total not designated as hedges, net of tax	31	88	(54)
Total gains (losses) on derivative instruments, net of tax	23	91	(57)
Cash Flow Hedging [Member]
Designated as cash flow or fair value hedges
Income tax	1	(2)	1
Total designated as cash flow or fair value hedges, net of tax	(8)	3	(3)
Cost of Sales [Member]
Not designated as hedges
Natural gas contracts - net fair value adjustments	(6)	(7)	2
Cost of Sales [Member] | Cash Flow Hedging [Member]
Designated as cash flow or fair value hedges
Natural gas contracts - net gain (loss) reclassified from OCI	(10)	4	(1)
Operation and Maintenance Expense [Member] | Cash Flow Hedging [Member]
Designated as cash flow or fair value hedges
Natural gas contracts - net gain (loss) reclassified from OCI	(1)	1	0
Interest Expense [Member] | Cash Flow Hedging [Member]
Designated as cash flow or fair value hedges
Natural gas contracts - net gain (loss) reclassified from OCI	2	0	(3)
Operating Revenues [Member]
Not designated as hedges
Natural gas contracts - net fair value adjustments	$ 56	$ 149	$ (90)