Fair Value Measurement - Financial Instruments Carried at Fair Value (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Current Assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 0	$ 5	$ 0	$ 5	$ 1	$ 4	$ 8	$ 14
Net realized and unrealized gains (losses) included in earnings	(1)	7	(1)	7
Transfers out of Level 3	0	(8)	0	(7)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	2	(3)	(9)
Net unrealized gains (losses) on derivatives still held included in earnings	0	3	(2)	5
Current Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	91		91			32
Long- Term Assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	4	1	4	1	0	0	2	2
Net realized and unrealized gains (losses) included in earnings	4	(1)	8	(1)
Transfers out of Level 3	0	0	(4)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Net unrealized gains (losses) on derivatives still held included in earnings	4	(1)	4	1
Long- Term Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	40		40			2
Long- Term Assets | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted Investments, at Fair Value	15		15			0
Current Liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(1)	(1)	(1)	(1)	(3)	1	(1)	0
Net realized and unrealized (losses) gains included in earnings	(1)	1	8	0
Transfers out of Level 3	(2)
Transfers out of Level 3		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(1)	1
Settlements			(8)	(1)
Net unrealized gains (losses) on derivatives still held included in earnings	1	0	(1)	(1)
Current Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	83		83			58
Long- Term Liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(1)	(5)	(1)	(5)	$ (4)	3	$ 0	$ 2
Net realized and unrealized (losses) gains included in earnings	3	(5)	2	(6)
Transfers out of Level 3	0
Transfers out of Level 3		0	0	3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Settlements			0	0
Net unrealized gains (losses) on derivatives still held included in earnings	3	$ (5)	1	$ (5)
Long- Term Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	29		29			20
Level 1 | Current Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	59		59			13
Level 1 | Long- Term Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	16		16			1
Level 1 | Long- Term Assets | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted Investments, at Fair Value	14		14			0
Level 1 | Current Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	37		37			15
Level 1 | Long- Term Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	13		13			2
Level 2 | Current Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	32		32			15
Level 2 | Long- Term Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	20		20			1
Level 2 | Long- Term Assets | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted Investments, at Fair Value	1		1			0
Level 2 | Current Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	45		45			42
Level 2 | Long- Term Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	15		15			15
Level 3 | Current Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	0		0			4
Level 3 | Long- Term Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	4		4			0
Level 3 | Long- Term Assets | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted Investments, at Fair Value	0		0			0
Level 3 | Current Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	1		1			1
Level 3 | Long- Term Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	$ 1		$ 1			$ (3)