Fair Value Measurements (Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	$ 117
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	4,604		4,192
Total liabilities	1,110		1,157
Fair Value, Measurements, Recurring | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	1,178		965
Derivative Liabilities	934		888
Fair Value, Measurements, Recurring | Interest Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	94		105
Derivative Liabilities	176		269
Fair Value, Measurements, Recurring | Equity Securities | Large cap | United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,933	[1]	1,718	[1]
Fair Value, Measurements, Recurring | Equity Securities | Large cap | Non United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	11	[1]	10	[1]
Fair Value, Measurements, Recurring | Equity Securities | Equity securities, other | United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	59	[1]	51	[1]
Fair Value, Measurements, Recurring | Fixed Income Securities | Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	317	[1]	332	[1]
Fair Value, Measurements, Recurring | Fixed Income Securities | U.S. Treasury securities and agency debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	478	[1]	458	[1]
Fair Value, Measurements, Recurring | Fixed Income Securities | State and municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	328	[1]	329	[1]
Fair Value, Measurements, Recurring | Fixed Income Securities | Debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	20	[1]	23	[1]
Fair Value, Measurements, Recurring | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	82	[1]	60	[1]
Fair Value, Measurements, Recurring | Restricted cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	104	[1]	141	[1]
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	2,365		2,100
Total liabilities	19		10
Fair Value, Measurements, Recurring | Level 1 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	54		44
Derivative Liabilities	19		10
Fair Value, Measurements, Recurring | Level 1 | Interest Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	0		0
Derivative Liabilities	0		0
Fair Value, Measurements, Recurring | Level 1 | Equity Securities | Large cap | United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,933	[1]	1,718	[1]
Fair Value, Measurements, Recurring | Level 1 | Equity Securities | Large cap | Non United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	11	[1]	10	[1]
Fair Value, Measurements, Recurring | Level 1 | Equity Securities | Equity securities, other | United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	59	[1]	51	[1]
Fair Value, Measurements, Recurring | Level 1 | Fixed Income Securities | Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 1 | Fixed Income Securities | U.S. Treasury securities and agency debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	308	[1]	277	[1]
Fair Value, Measurements, Recurring | Level 1 | Fixed Income Securities | State and municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 1 | Fixed Income Securities | Debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 1 | Restricted cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	2,122		1,999
Total liabilities	913		983
Fair Value, Measurements, Recurring | Level 2 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	1,007		828
Derivative Liabilities	737		714
Fair Value, Measurements, Recurring | Level 2 | Interest Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	94		105
Derivative Liabilities	176		269
Fair Value, Measurements, Recurring | Level 2 | Equity Securities | Large cap | United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 2 | Equity Securities | Large cap | Non United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 2 | Equity Securities | Equity securities, other | United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 2 | Fixed Income Securities | Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	317	[1]	332	[1]
Fair Value, Measurements, Recurring | Level 2 | Fixed Income Securities | U.S. Treasury securities and agency debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	170	[1]	181	[1]
Fair Value, Measurements, Recurring | Level 2 | Fixed Income Securities | State and municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	328	[1]	329	[1]
Fair Value, Measurements, Recurring | Level 2 | Fixed Income Securities | Debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	20	[1]	23	[1]
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	82	[1]	60	[1]
Fair Value, Measurements, Recurring | Level 2 | Restricted cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	104	[1]	141	[1]
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	117		93
Total liabilities	178		164
Fair Value, Measurements, Recurring | Level 3 | Commodity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	117		93
Derivative Liabilities	178		164
Fair Value, Measurements, Recurring | Level 3 | Interest Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets	0		0
Derivative Liabilities	0		0
Fair Value, Measurements, Recurring | Level 3 | Equity Securities | Large cap | United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 3 | Equity Securities | Large cap | Non United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 3 | Equity Securities | Equity securities, other | United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 3 | Fixed Income Securities | Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 3 | Fixed Income Securities | U.S. Treasury securities and agency debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 3 | Fixed Income Securities | State and municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 3 | Fixed Income Securities | Debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 3 | Cash equivalents and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Level 3 | Restricted cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 0	[1]	$ 0	[1]

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