Fair Value Measurements (Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Details) (USD $) In Millions	Jun. 30, 2011		Dec. 31, 2010
Total assets	$ 4,026		$ 4,221
Total liabilities	776		830
Level 1 [Member]
Total assets	2,274		2,092
Total liabilities	9		12
Level 1 [Member] | Cash and Cash Equivalents [Member]
Investments			25	[1]
Level 1 [Member] | Fixed Income Securities [Member] | U.S. Treasury Securities and Agency Debentures [Member]
Investments	348	[1]	228	[1]
Level 1 [Member] | Equity Securities [Member] | Large Cap [Member] | United States [Member]
Investments	1,819	[1]	1,709	[1]
Level 1 [Member] | Equity Securities [Member] | Large Cap [Member] | Non United States [Member]
Investments	12	[1]	12	[1]
Level 1 [Member] | Equity Securities [Member] | Other Debt Securities [Member] | United States [Member]
Investments	58	[1]	56	[1]
Level 1 [Member] | Commodity Contract [Member]
Derivatives	37		62
Derivatives	9		12
Level 2 [Member]
Total assets	1,670		2,082
Total liabilities	563		721
Level 2 [Member] | Cash and Cash Equivalents [Member]
Investments	70	[1]	97	[1]
Level 2 [Member] | Restricted Cash Equivalents [Member]
Investments	301	[1]	400	[1]
Level 2 [Member] | Fixed Income Securities [Member] | Corporate Debt Securities [Member]
Investments	291	[1]	327	[1]
Level 2 [Member] | Fixed Income Securities [Member] | State and Municipal [Member]
Investments	268	[1]	286	[1]
Level 2 [Member] | Fixed Income Securities [Member] | U.S. Treasury Securities and Agency Debentures [Member]
Investments	164	[1]	165	[1]
Level 2 [Member] | Fixed Income Securities [Member] | Other Debt Securities [Member]
Investments	22	[1]	19	[1]
Level 2 [Member] | Interest Rate Contract [Member]
Derivatives	85		54
Derivatives	34		5
Level 2 [Member] | Commodity Contract [Member]
Derivatives	469		734
Derivatives	529		716
Level 3 [Member]
Total assets	82		47
Total liabilities	204		97
Level 3 [Member] | Commodity Contract [Member]
Derivatives	82		47
Derivatives	204		97
Cash and Cash Equivalents [Member]
Investments	70	[1]	122	[1]
Restricted Cash Equivalents [Member]
Investments	301	[1]	400	[1]
Fixed Income Securities [Member] | Corporate Debt Securities [Member]
Investments	291	[1]	327	[1]
Fixed Income Securities [Member] | State and Municipal [Member]
Investments	268	[1]	286	[1]
Fixed Income Securities [Member] | U.S. Treasury Securities and Agency Debentures [Member]
Investments	512	[1]	393	[1]
Fixed Income Securities [Member] | Other Debt Securities [Member]
Investments	22	[1]	19	[1]
Equity Securities [Member] | Large Cap [Member] | United States [Member]
Investments	1,819	[1]	1,709	[1]
Equity Securities [Member] | Large Cap [Member] | Non United States [Member]
Investments	12	[1]	12	[1]
Equity Securities [Member] | Other Debt Securities [Member] | United States [Member]
Investments	58	[1]	56	[1]
Interest Rate Contract [Member]
Derivatives	85		54
Derivatives	34		5
Commodity Contract [Member]
Derivatives	588		843
Derivatives	$ 742		$ 825

[1] Includes investments held in the nuclear decommissioning and rabbi trusts.