Financial Risk Management - Netting Arrangements on Physical and Financial Derivative Assets and (Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Derivatives Fair Value [Line Items]
Gross Amounts, Assets	$ 119
Gross Amounts of Offset in the Consolidated Balance Sheet Physical Derivative and Financial Instruments Assets	(14)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	105
Net Amounts, Assets	105
Gross Amounts, Liabilities	(18)
Physical Derivative and Financial Instruments, Liabilities	14
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(4)
Net Amounts, Liabilities	(4)
Commodity [Member]
Derivatives Fair Value [Line Items]
Gross Amounts, Assets	81
Gross Amounts of Offset in the Consolidated Balance Sheet Physical Derivative and Financial Instruments Assets	(12)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	69
Net Amounts, Assets	69
Gross Amounts, Liabilities	(15)
Physical Derivative and Financial Instruments, Liabilities	12
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(3)
Net Amounts, Liabilities	(3)
Interest Rate and Other [Member]
Derivatives Fair Value [Line Items]
Gross Amounts, Assets	38
Gross Amounts of Offset in the Consolidated Balance Sheet Physical Derivative and Financial Instruments Assets	(1)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	37
Net Amounts, Assets	37
Counterparty netting [Member]
Derivatives Fair Value [Line Items]
Gross Amounts of Offset in the Consolidated Balance Sheet Physical Derivative and Financial Instruments Assets	(1)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	(1)
Net Amounts, Assets	(1)
Physical Derivative and Financial Instruments, Liabilities	1
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	1
Net Amounts, Liabilities	1
Other [Member]
Derivatives Fair Value [Line Items]
Gross Amounts, Liabilities	(3)
Physical Derivative and Financial Instruments, Liabilities	1
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(2)
Net Amounts, Liabilities	$ (2)