Fair Value Measurements and Disclosures of Assets and Liabilities Measured on a Recurring Basis (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Financial Assets [Abstract]
Mutual Fund Investments	$ 112		$ 111
Commodity Derivative Instruments	830		890
Financial Liabilities [Abstract]
Commodity Derivative Instruments	0		0
Portion of Deferred Compensation Liability Measured at Fair Value	(137)		(134)
Quoted Prices in Active Markets (Level 1)
Financial Assets [Abstract]
Mutual Fund Investments	112	[1]	111	[1]
Commodity Derivative Instruments	0	[1]	0	[1]
Financial Liabilities [Abstract]
Commodity Derivative Instruments	0	[1]	0	[1]
Portion of Deferred Compensation Liability Measured at Fair Value	(137)	[1]	(134)	[1]
Significant Other Observable Inputs (Level 2)
Financial Assets [Abstract]
Mutual Fund Investments	0	[2]	0	[2]
Commodity Derivative Instruments	832	[2]	890	[2]
Financial Liabilities [Abstract]
Commodity Derivative Instruments	(2)	[2]	0	[2]
Portion of Deferred Compensation Liability Measured at Fair Value	0	[2]	0	[2]
Significant Unobservable Inputs (Level 3)
Financial Assets [Abstract]
Mutual Fund Investments	0	[3]	0	[3]
Commodity Derivative Instruments	0	[3]	0	[3]
Financial Liabilities [Abstract]
Commodity Derivative Instruments	0	[3]	0	[3]
Portion of Deferred Compensation Liability Measured at Fair Value	0	[3]	0	[3]
Adjustment
Financial Assets [Abstract]
Mutual Fund Investments	0	[4]	0	[4]
Commodity Derivative Instruments	(2)	[4]	0	[4]
Financial Liabilities [Abstract]
Commodity Derivative Instruments	2	[4]	0	[4]
Portion of Deferred Compensation Liability Measured at Fair Value	$ 0	[4]	$ 0	[4]

[1] Level 1 measurements are fair value measurements which use quoted market prices (unadjusted) in active markets for identical assets or liabilities. We use Level 1 inputs when available as Level 1 inputs generally provide the most reliable evidence of fair value.

[2] Level 2 measurements are fair value measurements which use inputs, other than quoted prices included within Level 1, which are observable for the asset or liability, either directly or indirectly.

[3] Level 3 measurements are fair value measurements which use unobservable inputs.

[4] Amount represents the impact of netting provisions within our master agreements that allow us to net cash settle asset and liability positions with the same counterparty.