Derivative Instruments - Fair Value Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	$ 68	$ 357
Derivative assets Netting	(14)	(127)
Derivative assets, Collateral	0	0
Derivative assets	54	230
Gross derivative liabilities	(1,630)	(1,378)
Derivative liabilities Netting	14	127
Derivative liabilities, Collateral	264	68
Derivative liabilities	(1,352)	(1,183)
Commodity derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	37	301
Derivative assets Netting	(14)	(127)
Derivative assets, Collateral	0	0
Derivative assets	23	174
Gross derivative liabilities	(15)	(132)
Derivative liabilities Netting	14	127
Derivative liabilities, Collateral	0	2
Derivative liabilities	(1)	(3)
Interest-rate derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	31	56
Derivative assets Netting	0	0
Derivative assets, Collateral	0	0
Derivative assets	31	56
Gross derivative liabilities	(1,512)	(1,238)
Derivative liabilities Netting	0	0
Derivative liabilities, Collateral	264	66
Derivative liabilities	(1,248)	(1,172)
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	1
Gross derivative liabilities	0	(2)
Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	68	356
Gross derivative liabilities	(1,630)	(1,376)
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	1
Gross derivative liabilities	0	(2)
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	37	300
Gross derivative liabilities	(15)	(130)
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	31	56
Gross derivative liabilities	(1,512)	(1,238)
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
WES [Member] | Interest-rate derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities Netting	0	0
Derivative liabilities, Collateral	0	0
Derivative liabilities	(103)	(8)
WES [Member] | 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
WES [Member] | 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	(103)	(8)
WES [Member] | 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