Long-term Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
General mortgage bonds used as collateral	$ 408	$ 408
Bonds General Mortgage Due 2044 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	600
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Dec. 31, 2044
Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	300	300
Line of Credit Facility, Interest Rate Description	London Interbank Offered Rate plus 112.5
Loans Outstanding	0	0
Letters Of Credit Outstanding	4	4
Percentage on limitation of debt to total capitalization under covenant (in hundredths)	65.00%
January 2014 [Member] | Bonds Pollution Control Due2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.25%
Extinguishment Of Debt Purchase Price Percentage Of Principal	101.00%
Debt Instrument, Maturity Date	Dec. 31, 2017
Extinguishment of Debt, Amount	44
January 2014 [Member] | Bonds Pollution Control Due2027 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.60%
Extinguishment Of Debt Purchase Price Percentage Of Principal	101.00%
Debt Instrument, Maturity Date	Dec. 31, 2027
Extinguishment of Debt, Amount	56
April 2014 [Member] | Bonds Pollution Control Due2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.25%
Extinguishment Of Debt Purchase Price Percentage Of Principal	100.00%
Debt Instrument, Maturity Date	Dec. 31, 2017
Extinguishment of Debt, Amount	$ 84