Derivative Instruments - Fair Value Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		$ 121	$ 526
Gross derivative liabilities		(1,904)	(1,722)
Derivative assets netting	[1]	(14)	(178)
Derivative liabilities netting	[1]	14	178
Cash collateral from counterparties		0	(8)
Cash collateral held by counterparties		424	58
Derivative assets		107	340
Derivative liabilities		(1,466)	(1,486)
Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		99	470
Gross derivative liabilities		(50)	(180)
Derivative assets netting	[1]	(14)	(178)
Derivative liabilities netting	[1]	14	178
Cash collateral from counterparties		0	(8)
Cash collateral held by counterparties		2	0
Derivative assets		85	284
Derivative liabilities		(34)	(2)
Interest-Rate Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		22	56
Gross derivative liabilities		(1,854)	(1,542)
Derivative assets netting	[1]	0	0
Derivative liabilities netting	[1]	0	0
Cash collateral from counterparties		0	0
Cash collateral held by counterparties		422	58
Derivative assets		22	56
Derivative liabilities		(1,432)	(1,484)
Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		0	10
Gross derivative liabilities		(1)	(1)
Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		121	516
Gross derivative liabilities		(1,903)	(1,721)
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] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		0	10
Gross derivative liabilities		(1)	(1)
Recurring [Member] | Commodity Derivatives [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		99	460
Gross derivative liabilities		(49)	(179)
Recurring [Member] | Commodity Derivatives [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 assets		0	0
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 assets		22	56
Gross derivative liabilities		(1,854)	(1,542)
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 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.