Derivative Instruments - Fair Value Table (Detail) (Not Designated as Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	$ 493		$ 380
Gross derivative liabilities	(1,455)		(986)
Derivative assets netting	(189)	[1]	(279)	[1]
Derivative liabilities netting	189	[1]	279	[1]
Cash collateral from counterparties	(13)		0
Cash collateral held by counterparties	23		7
Derivative assets	291		101
Derivative liabilities	(1,243)		(700)
Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	493		380
Gross derivative liabilities	(238)		(332)
Interest-Rate and Other Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	(1,217)		(654)
Derivative liabilities netting	0	[1]	0	[1]
Cash collateral held by counterparties	0		0
Derivative liabilities	(1,217)		(654)
Financial Institutions [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(187)	[1]	(153)	[1]
Derivative liabilities netting	187	[1]	153	[1]
Cash collateral from counterparties	(13)		0
Cash collateral held by counterparties	23		7
Derivative assets	271		58
Derivative liabilities	(24)		(40)
Other Counterparties [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(2)	[1]	(126)	[1]
Derivative liabilities netting	2	[1]	126	[1]
Cash collateral from counterparties	0		0
Cash collateral held by counterparties	0		0
Derivative assets	20		43
Derivative liabilities	(2)		(6)
Fair Value, Inputs, Level 1 [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	0		0
Fair Value, Inputs, Level 1 [Member] | Interest-Rate and Other Derivatives [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative liabilities	0		0
Fair Value, Inputs, Level 1 [Member] | Financial Institutions [Member] | Commodity Derivatives [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	0		0
Fair Value, Inputs, Level 1 [Member] | Other Counterparties [Member] | Commodity Derivatives [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	0		0
Fair Value, Inputs, Level 2 [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	493		380
Gross derivative liabilities	(1,455)		(986)
Fair Value, Inputs, Level 2 [Member] | Interest-Rate and Other Derivatives [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative liabilities	(1,217)		(654)
Fair Value, Inputs, Level 2 [Member] | Financial Institutions [Member] | Commodity Derivatives [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	471		211
Gross derivative liabilities	(234)		(200)
Fair Value, Inputs, Level 2 [Member] | Other Counterparties [Member] | Commodity Derivatives [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	22		169
Gross derivative liabilities	(4)		(132)
Fair Value, Inputs, Level 3 [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	0		0
Fair Value, Inputs, Level 3 [Member] | Interest-Rate and Other Derivatives [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative liabilities	0		0
Fair Value, Inputs, Level 3 [Member] | Financial Institutions [Member] | Commodity Derivatives [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	0		0
Fair Value, Inputs, Level 3 [Member] | Other Counterparties [Member] | Commodity Derivatives [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	$ 0		$ 0

[1] Represents the impact of netting commodity derivative assets and liabilities with counterparties where the Company has the contractual right and intends to net settle.