Financial Risk Management - Fair Values for Corporation's Net Physical Derivative and Financial Assets and (Liabilities) (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Disclosure - Financial Risk Management - Fair Values for Corporation's Net Physical Derivative and Financial Assets and (Liabilities) (Parenthetical) (Detail) [Line Items]
Long-term debt, including current maturities	$ 5,980
Fair Value, Inputs, Level 2
Disclosure - Financial Risk Management - Fair Values for Corporation's Net Physical Derivative and Financial Assets and (Liabilities) (Parenthetical) (Detail) [Line Items]
Long-term Debt, Fair Value	6,938
Derivative contracts designated as hedging instruments	1
Derivative contracts, designated and not designated, as hedging instruments	105
Fair Value, Inputs, Level 2 | Commodity [Member]
Disclosure - Financial Risk Management - Fair Values for Corporation's Net Physical Derivative and Financial Assets and (Liabilities) (Parenthetical) (Detail) [Line Items]
Derivative contracts, designated and not designated, as hedging instruments	69
Fair Value, Inputs, Level 2 | Interest Rate and Other [Member]
Disclosure - Financial Risk Management - Fair Values for Corporation's Net Physical Derivative and Financial Assets and (Liabilities) (Parenthetical) (Detail) [Line Items]
Derivative contracts, designated and not designated, as hedging instruments	36
Fair Value, Inputs, Level 1
Disclosure - Financial Risk Management - Fair Values for Corporation's Net Physical Derivative and Financial Assets and (Liabilities) (Parenthetical) (Detail) [Line Items]
Derivative contracts not designated as hedging instruments	$ 3