Quantitative Disclosures About Unobservable Inputs For Level 3 Fair Value Measurements (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Oil Trades
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	(805)	[1]
Oil Trades | Oil Price Volatility Curve
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unobservable Input	Oil price volatility curve	[1]
Weighted average of price volatility curve, percentage	22.44%	[1]
Oil Trades | Oil Price Volatility Curve | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price volatility curve, percentage	14.79%	[1]
Oil Trades | Oil Price Volatility Curve | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price volatility curve, percentage	32.61%	[1]
Natural Gas Trades
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	(251)	[1]
Natural Gas Trades | Natural Gas Price Volatility Curve
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unobservable Input	Natural gas price volatility curve	[1]
Weighted average of price volatility curve, percentage	23.51%	[1]
Natural Gas Trades | Natural Gas Price Volatility Curve | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price volatility curve, percentage	21.61%	[1]
Natural Gas Trades | Natural Gas Price Volatility Curve | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price volatility curve, percentage	41.21%	[1]
Natural Gas Basis Swaps
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	(17)	[2]
Natural Gas Basis Swaps | Physical Pricing Point Forward Curves
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unobservable Input	Physical pricing point forward curves	[2]
Weighted average of price forward curve	(0.0021)	[2]
Natural Gas Basis Swaps | Physical Pricing Point Forward Curves | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price forward curve	(1.87)	[2]
Natural Gas Basis Swaps | Physical Pricing Point Forward Curves | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price forward curve	0.05	[2]
Interest Rate Swaption | Physical Pricing Point Forward Curves
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Weighted average of price volatility curve, percentage	1.46%
Interest Rate Swaption | Physical Pricing Point Forward Curves | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price volatility curve, percentage	0.30%
Interest Rate Swaption | Physical Pricing Point Forward Curves | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price volatility curve, percentage	2.94%
Interest Rate Volatility | Physical Pricing Point Forward Curves
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Weighted average of price volatility curve, percentage	43.25%
Interest Rate Volatility | Physical Pricing Point Forward Curves | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price volatility curve, percentage	43.14%
Interest Rate Volatility | Physical Pricing Point Forward Curves | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Price volatility curve, percentage	43.35%
Interest Rate Swap
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	(2)

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

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