Derivative Instruments and Hedging Activities (Balance Sheet Disclosures) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	$ 46		$ 40
Derivative Assets, Long-term	52		59
Derivative Assets	98		99
Derivative Liabilities, Short-term	0		1
Derivative Liabilities, Long-term	114		90
Derivative Liabilities	114		91
Net Assets (Liabilities)	(16)		8
Cash Flow Hedges | Commodity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	24		41
Derivative Assets, Long-term	2		1
Derivative Assets	26		42
Derivative Liabilities, Short-term	1		2
Derivative Liabilities, Long-term	3		3
Derivative Liabilities	4		5
Net Assets (Liabilities)	22		37
Cash Flow Hedges | Interest rate contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	0		0
Derivative Assets, Long-term	0		0
Derivative Assets	0		0
Derivative Liabilities, Short-term	0		0
Derivative Liabilities, Long-term	114		90
Derivative Liabilities	114		90
Net Assets (Liabilities)	(114)		(90)
Economic Hedges
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	28		31
Derivative Assets, Long-term	2		1
Derivative Assets	30		32
Derivative Liabilities, Short-term	24		26
Derivative Liabilities, Long-term	2		1
Derivative Liabilities	26		27
Net Assets (Liabilities)	4		5
Trading Activities
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	329		276
Derivative Assets, Long-term	153		142
Derivative Assets	482		418
Derivative Liabilities, Short-term	272		232
Derivative Liabilities, Long-term	92		79
Derivative Liabilities	364		311
Net Assets (Liabilities)	118		107
Trading and Non-Trading Activities
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	381		348
Derivative Assets, Long-term	157		144
Derivative Assets	538		492
Derivative Liabilities, Short-term	297		260
Derivative Liabilities, Long-term	211		173
Derivative Liabilities	508		433
Net Assets (Liabilities)	30		59
Netting and Collateral Received
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-term	(335)	[1]	(308)	[1]
Derivative Assets, Long-term	(105)	[1]	(85)	[1]
Derivative Assets	(440)	[1]	(393)	[1]
Derivative Liabilities, Short-term	(297)	[1]	(259)	[1]
Derivative Liabilities, Long-term	(97)	[1]	(83)	[1]
Derivative Liabilities	(394)	[1]	(342)	[1]
Net Assets (Liabilities)	$ (46)	[1]	$ (51)	[1]

[1] Netting of derivative receivables and derivative payables and the related cash collateral received and paid is permitted when a legally enforceable master netting agreement exists with a derivative counterparty.