Fair Value Measurements (Quantitative Disclosures About Unobservable Inputs For Level 3 Fair Value Measurements) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Oil Trades [Member]
Fair Value	$ (1,410)	[1]
Natural Gas Trades [Member]
Fair Value	(255)	[1]
Gas Basis Swaps [Member]
Fair Value	(40)	[2]
Interest Rate Swaptions [Member]
Fair Value	$ (3)	[1]
NYMEX Oil Price Forward Curve [Member] | Oil Trades [Member]
Unobservable Input	NYMEX oil price forward curve	[1]
Weighted average of price forward curve	$ 97.95	[1]
NYMEX Oil Price Forward Curve [Member] | Oil Trades [Member] | Maximum [Member]
Price forward curve	$ 105.33	[1]
NYMEX Oil Price Forward Curve [Member] | Oil Trades [Member] | Minimum [Member]
Price forward curve	$ 91.47	[1]
Oil Price Volatility Curve [Member] | Oil Trades [Member]
Unobservable Input	Oil price volatility curve	[1]
Weighted average of price volatility curve, percentage	23.16%	[1]
Oil Price Volatility Curve [Member] | Oil Trades [Member] | Maximum [Member]
Price volatility curve, percentage	31.03%	[1]
Oil Price Volatility Curve [Member] | Oil Trades [Member] | Minimum [Member]
Price volatility curve, percentage	16.39%	[1]
NYMEX Natural Gas Price Forward Curve [Member] | Natural Gas Trades [Member]
Unobservable Input	NYMEX natural gas price forward curve	[1]
Weighted average of price forward curve	$ 3.91	[1]
NYMEX Natural Gas Price Forward Curve [Member] | Natural Gas Trades [Member] | Maximum [Member]
Price forward curve	$ 5.81	[1]
NYMEX Natural Gas Price Forward Curve [Member] | Natural Gas Trades [Member] | Minimum [Member]
Price forward curve	$ 2.13	[1]
Natural Gas Price Volatility Curve [Member] | Natural Gas Trades [Member]
Unobservable Input	Natural gas price volatility curve	[1]
Weighted average of price volatility curve, percentage	28.27%	[1]
Natural Gas Price Volatility Curve [Member] | Natural Gas Trades [Member] | Maximum [Member]
Price volatility curve, percentage	53.67%	[1]
Natural Gas Price Volatility Curve [Member] | Natural Gas Trades [Member] | Minimum [Member]
Price volatility curve, percentage	21.40%	[1]
NYMEX Natural Gas Forward Curve [Member] | Gas Basis Swaps [Member]
Unobservable Input	NYMEX natural gas forward curve	[2]
Weighted average of price forward curve	$ 3.22	[2]
NYMEX Natural Gas Forward Curve [Member] | Gas Basis Swaps [Member] | Maximum [Member]
Price forward curve	$ 5.06	[2]
NYMEX Natural Gas Forward Curve [Member] | Gas Basis Swaps [Member] | Minimum [Member]
Price forward curve	$ 1.95	[2]
Physical Pricing Point Forward Curves [Member] | Gas Basis Swaps [Member]
Unobservable Input	Physical pricing point forward curves	[2]
Weighted average of price forward curve	$ (0.22)	[2]
Physical Pricing Point Forward Curves [Member] | Gas Basis Swaps [Member] | Maximum [Member]
Price forward curve	$ 0.20	[2]
Physical Pricing Point Forward Curves [Member] | Gas Basis Swaps [Member] | Minimum [Member]
Price forward curve	$ 1.29	[2]
Forward Interest Rate Curve [Member] | Interest Rate Swaptions [Member]
Unobservable Input	Forward interest rate curve	[1]
Weighted average of price volatility curve, percentage	1.80%	[1]
Forward Interest Rate Curve [Member] | Interest Rate Swaptions [Member] | Maximum [Member]
Price volatility curve, percentage	3.66%	[1]
Forward Interest Rate Curve [Member] | Interest Rate Swaptions [Member] | Minimum [Member]
Price volatility curve, percentage	0.41%	[1]
Interest Rate Volatility [Member] | Interest Rate Swaptions [Member]
Unobservable Input	Interest rate volatility	[1]
Weighted average of price volatility curve, percentage	47.40%	[1]
Interest Rate Volatility [Member] | Interest Rate Swaptions [Member] | Maximum [Member]
Price volatility curve, percentage	52.07%	[1]
Interest Rate Volatility [Member] | Interest Rate Swaptions [Member] | Minimum [Member]
Price volatility curve, percentage	40.04%	[1]

[1] Fair value is based on an estimate derived from option models.

[2] Fair value is based on an estimate of discounted cash flows.