Fair Value Measurement - Financial Instruments Carried at Fair Value (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 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	$ 1	$ 8	$ 1	$ 8	$ 8	$ 4	$ 5	$ 14
Net realized and unrealized gains (losses) included in earnings	(7)	10	0	8
Transfers out of Level 3	0	(6)		(8)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	1	(3)	(6)
Net unrealized gains (losses) on derivatives still held included in earnings	(3)	7	1	7
Current Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	98		98			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	0	2	0	2	1	0	1	2
Net realized and unrealized gains (losses) included in earnings	0	1	0	2
Transfers out of Level 3	1	0		(2)
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	0	1	0	1
Long- Term Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	29		29			2
Long- Term Assets | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted Investments, at Fair Value	6		6			0
Current Liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	(3)	(1)	(3)	(1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(3)	(1)	(3)	(1)	3	(1)	1	0
Net realized and unrealized (losses) gains included in earnings	2	1	6	(1)
Transfers out of Level 3	0
Transfers out of Level 3		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	2	1
Settlements			(8)	0
Net unrealized gains (losses) on derivatives still held included in earnings	(1)	0	(2)	(1)
Current Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	66		66			58
Long- Term Liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	(4)	0	(4)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(4)	0	(4)	0	$ 1	(3)	$ 1	$ (2)
Net realized and unrealized (losses) gains included in earnings	(3)	1	(1)	1
Transfers out of Level 3	0
Transfers out of Level 3		0		1
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)	$ 0	1	$ 0
Long- Term Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	37		37			20
Level 1 | Current Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	74		74			13
Level 1 | Long- Term Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	24		24			1
Level 1 | Long- Term Assets | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Restricted Investments, at Fair Value	6		6			0
Level 1 | Current Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	41		41			15
Level 1 | Long- Term Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	17		17			2
Level 2 | Current Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	23		23			15
Level 2 | Long- Term Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	5		5			1
Level 2 | 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 2 | Current Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	22		22			42
Level 2 | Long- Term Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	16		16			15
Level 3 | Current Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	1		1			4
Level 3 | Long- Term Assets | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - assets	0		0			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	3		3			1
Level 3 | Long- Term Liabilities | Commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivatives - liabilities	$ 4		$ 4			$ (3)