Financial Risk Management and Trading Activities - Netting Arrangements on Physical and Financial Derivative Assets and (Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivatives Fair Value [Line Items]
Gross Amounts, Assets	$ 3,225	$ 3,137
Physical Derivative and Financial Instruments, Assets	(2,861)	(2,083)
Cash Collateral, Assets	(33)	(79)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	331	975
Gross Amounts Not Offset in the Consolidated Balance Sheet, Assets	(3)	(44)
Net Amounts, Assets	328	931
Gross Amounts, Liabilities	(3,220)	(3,224)
Physical Derivative and Financial Instruments, Liabilities	2,861	2,083
Cash Collateral, Liabilities	98	168
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(261)	(973)
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities	3	44
Net Amounts, Liabilities	(258)	(929)
Commodity [Member]
Derivatives Fair Value [Line Items]
Gross Amounts, Assets	3,137	3,086
Physical Derivative and Financial Instruments, Assets	(2,797)	(1,867)
Cash Collateral, Assets	(31)	(79)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	309	1,140
Gross Amounts Not Offset in the Consolidated Balance Sheet, Assets	(1)	(41)
Net Amounts, Assets	308	1,099
Gross Amounts, Liabilities	(3,207)	(3,212)
Physical Derivative and Financial Instruments, Liabilities	2,797	1,867
Cash Collateral, Liabilities	98	168
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(312)	(1,177)
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities	1	41
Net Amounts, Liabilities	(311)	(1,136)
Interest Rate and Other [Member]
Derivatives Fair Value [Line Items]
Gross Amounts, Assets	88	51
Physical Derivative and Financial Instruments, Assets	(7)	(10)
Cash Collateral, Assets	(2)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	79	41
Gross Amounts Not Offset in the Consolidated Balance Sheet, Assets	(2)	(3)
Net Amounts, Assets	77	38
Counterparty netting [Member]
Derivatives Fair Value [Line Items]
Physical Derivative and Financial Instruments, Assets	(57)	(206)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	(57)	(206)
Net Amounts, Assets	(57)	(206)
Physical Derivative and Financial Instruments, Liabilities	57	206
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	57	206
Net Amounts, Liabilities	57	206
Other [Member]
Derivatives Fair Value [Line Items]
Gross Amounts, Liabilities	(13)	(12)
Physical Derivative and Financial Instruments, Liabilities	7	10
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(6)	(2)
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities	2	3
Net Amounts, Liabilities	$ (4)	$ 1