Long-Term Debt (Narrative) (Details) - USD ($)	1 Months Ended	6 Months Ended
	Mar. 31, 2015	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Long-Term Debt [Line Items]
Repayments of long-term debt		$ 569,000,000	$ 65,000,000
Note redemption price to principal amount, percentage		100.00%
Proceeds from issuance of long-term debt		$ 725,000,000	$ 250,000,000
Percentage of principal amount plus accrued and unpaid interest and make-whole premium		100.00%
Increase in basis points		0.50%
Percentage of fair value of cost of property additions certified to the Deed of Trust collateral agent		85.00%
Estimated fair value of our long-term debt including current maturities		$ 6,622,000,000		$ 6,844,000,000
Carrying amount		5,791,000,000		5,636,000,000
Available bond credits		2,637,000,000
Future debt subject to property additions to the Deed of Trust		1,103,000,000
6.375% Fixed Senior Notes Due January 15, 2015 [Member]
Long-Term Debt [Line Items]
Aggregate principal amount		$ 500,000,000		500,000,000
Interest percentage		6.375%
Transition Bond [Member]
Long-Term Debt [Line Items]
Debt principal amount		$ 69,000,000
2.950% Fixed Senior Notes due April 1, 2025 [Member]
Long-Term Debt [Line Items]
Aggregate principal amount	$ 350,000,000	$ 350,000,000
Interest percentage	2.95%	2.95%
2.150% Fixed Senior Notes Due June 1, 2019 [Member]
Long-Term Debt [Line Items]
Aggregate principal amount		$ 250,000,000		250,000,000
Interest percentage		2.15%
Proceeds from issuance of long-term debt	$ 714,000,000
4.550% Fixed Senior Notes Due December 1, 2041 [Member]
Long-Term Debt [Line Items]
Aggregate principal amount		$ 400,000,000		$ 400,000,000
Interest percentage		4.55%
3.750% Fixed Senior Notes due April 1, 2045 [Member]
Long-Term Debt [Line Items]
Aggregate principal amount	$ 375,000,000	$ 375,000,000
Interest percentage	3.75%	3.75%