Fair Value Measurements (Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Assets:
Nuclear decommissioning trust	$ 737		$ 654
Equity securities [Member]
Assets:
Nuclear decommissioning trust	471	[1]	393	[1]
Debt securities [Member]
Assets:
Nuclear decommissioning trust	266	[2]	261	[2]
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Total assets	439		429
Liabilities:
Total liabilities	129		208
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity securities [Member]
Assets:
Nuclear decommissioning trust	299	[3]	249
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Debt securities [Member]
Assets:
Nuclear decommissioning trust			0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodity contracts [Member]
Assets:
Derivative Assets	140		180
Liabilities:
Derivative Liabilities	129		208
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Interest rate swaps [Member]
Assets:
Derivative Assets	0		0
Liabilities:
Derivative Liabilities	0		0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Total assets	1,380		2,191
Liabilities:
Total liabilities	26		2,188
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Equity securities [Member]
Assets:
Nuclear decommissioning trust	172	[3]	144
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Debt securities [Member]
Assets:
Nuclear decommissioning trust	266		261
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodity contracts [Member]
Assets:
Derivative Assets	941		1,784
Liabilities:
Derivative Liabilities	26		121
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Interest rate swaps [Member]
Assets:
Derivative Assets	1		2
Liabilities:
Derivative Liabilities			2,067
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Total assets	48	[4]	83	[4]
Liabilities:
Total liabilities	1,219	[4]	54	[4]
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Equity securities [Member]
Assets:
Nuclear decommissioning trust	0	[3],[4]	0	[4]
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Debt securities [Member]
Assets:
Nuclear decommissioning trust	0	[4]	0	[4]
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commodity contracts [Member]
Assets:
Derivative Assets	48	[4]	83	[4]
Liabilities:
Derivative Liabilities	52	[4]	54	[4]
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Interest rate swaps [Member]
Assets:
Derivative Assets	0	[4]	0	[4]
Liabilities:
Derivative Liabilities	1,167	[4]	0	[4]
Fair Value, Measurements, Recurring [Member] | Reclassification
Assets:
Total assets	2	[5]
Liabilities:
Total liabilities	2	[5]
Fair Value, Measurements, Recurring [Member] | Reclassification | Equity securities [Member]
Assets:
Nuclear decommissioning trust		[3],[5]
Fair Value, Measurements, Recurring [Member] | Reclassification | Debt securities [Member]
Assets:
Nuclear decommissioning trust		[5]
Fair Value, Measurements, Recurring [Member] | Reclassification | Commodity contracts [Member]
Assets:
Derivative Assets	2	[5]
Liabilities:
Derivative Liabilities	2	[5]
Fair Value, Measurements, Recurring [Member] | Reclassification | Interest rate swaps [Member]
Assets:
Derivative Assets	0	[5]
Liabilities:
Derivative Liabilities		[5]
Fair Value, Measurements, Recurring [Member] | Total [Member]
Assets:
Total assets	1,869		2,703
Liabilities:
Total liabilities	1,376		2,450
Fair Value, Measurements, Recurring [Member] | Total [Member] | Equity securities [Member]
Assets:
Nuclear decommissioning trust	471	[3]	393
Fair Value, Measurements, Recurring [Member] | Total [Member] | Debt securities [Member]
Assets:
Nuclear decommissioning trust	266		261
Fair Value, Measurements, Recurring [Member] | Total [Member] | Commodity contracts [Member]
Assets:
Derivative Assets	1,131		2,047
Liabilities:
Derivative Liabilities	209		383
Fair Value, Measurements, Recurring [Member] | Total [Member] | Interest rate swaps [Member]
Assets:
Derivative Assets	1		2
Liabilities:
Derivative Liabilities	$ 1,167		$ 2,067

[1] The investment objective for equity securities is to invest tax efficiently and to match the performance of the S&P 500 Index.

[2] The investment objective for debt securities is to invest in a diversified tax efficient portfolio with an overall portfolio rating of AA or above as graded by S&P or Aa2 by Moody's Investors Services, Inc. The debt securities are heavily weighted with municipal bonds. The debt securities had an average coupon rate of 3.97% and 4.38% at September 30, 2013 and December 31, 2012, respectively, and an average maturity of 6 years at both September 30, 2013 and December 31, 2012.

[3] The nuclear decommissioning trust investment is included in the investments line in the balance sheet. See Note 12.

[4] See table below for description of Level 3 assets and liabilities.

[5] Includes unrealized mark-to-market net gain (loss) as well as the net realized effect on interest paid/accrued, both reported in "Interest Expense and Related Charges" (see Note 12).