Derivative Instruments - Fair Value Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		$ 315	$ 493
Gross derivative liabilities		(1,651)	(1,455)
Derivative assets netting	[1]	(118)	(189)
Derivative liabilities netting	[1]	118	189
Cash collateral from counterparties		(6)	(13)
Cash collateral held by counterparties		67	23
Derivative assets		191	291
Derivative liabilities		(1,466)	(1,243)
Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		315	493
Gross derivative liabilities		(133)	(238)
Commodity Derivatives [Member] | Financial Institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	[1]	(115)	(187)
Derivative liabilities netting	[1]	115	187
Cash collateral from counterparties		(6)	(13)
Cash collateral held by counterparties		0	0
Derivative assets		171	271
Derivative liabilities		(11)	(47)
Commodity Derivatives [Member] | Other Counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	[1]	(3)	(2)
Derivative liabilities netting	[1]	3	2
Cash collateral from counterparties		0	0
Cash collateral held by counterparties		0	0
Derivative assets		20	20
Derivative liabilities		(4)	(2)
Interest-Rate Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		0	0
Gross derivative liabilities		(1,518)	(1,217)
Derivative liabilities netting	[1]	0	0
Cash collateral held by counterparties		67	23
Derivative liabilities		(1,451)	(1,194)
Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		4	0
Gross derivative liabilities		0	0
Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		311	493
Gross derivative liabilities		(1,651)	(1,455)
Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		0	0
Gross derivative liabilities		0	0
Recurring [Member] | Commodity Derivatives [Member] | Financial Institutions [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		4	0
Gross derivative liabilities		0	0
Recurring [Member] | Commodity Derivatives [Member] | Financial Institutions [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		288	471
Gross derivative liabilities		(126)	(234)
Recurring [Member] | Commodity Derivatives [Member] | Financial Institutions [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		0	0
Gross derivative liabilities		0	0
Recurring [Member] | Commodity Derivatives [Member] | Other Counterparties [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		0	0
Gross derivative liabilities		0	0
Recurring [Member] | Commodity Derivatives [Member] | Other Counterparties [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		23	22
Gross derivative liabilities		(7)	(4)
Recurring [Member] | Commodity Derivatives [Member] | Other Counterparties [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		0	0
Gross derivative liabilities		0	0
Recurring [Member] | Interest-Rate Derivatives [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative liabilities		0	0
Recurring [Member] | Interest-Rate Derivatives [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative liabilities		(1,518)	(1,217)
Recurring [Member] | Interest-Rate Derivatives [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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.