Fair Value Measurement - Financial Instruments Carried at Fair Value (Detail) - USD ($) $ in Millions | 3 Months Ended | | |
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019 |
Current Assets | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value | $ 0 | $ 8 | $ 0 | $ 4 |
Net realized and unrealized gains (losses) included in earnings | 0 | 8 | | |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements | 0 | (4) | | |
Net unrealized gains (losses) on derivatives still held included in earnings | 0 | 7 | | |
Current Assets | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - assets | 58 | | 63 | |
Current Assets | Equity Securities [Member] | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Restricted Investments, at Fair Value | 2 | | 0 | |
Long- Term Assets | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value | 2 | 1 | 2 | 0 |
Net realized and unrealized gains (losses) included in earnings | 0 | 1 | | |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements | 0 | | | |
Net unrealized gains (losses) on derivatives still held included in earnings | 0 | 0 | | |
Long- Term Assets | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - assets | 17 | | 16 | |
Long- Term Assets | Equity Securities [Member] | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Restricted Investments, at Fair Value | 25 | | 23 | |
Current Liabilities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value | (4) | (3) | (3) | (1) |
Net realized and unrealized (losses) gains included in earnings | (5) | 5 | | |
Settlements | 4 | (7) | | |
Net unrealized gains (losses) on derivatives still held included in earnings | (3) | (2) | | |
Current Liabilities | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - liabilities | 77 | | 56 | |
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) | (1) | (1) | $ (3) |
Net realized and unrealized (losses) gains included in earnings | 0 | 2 | | |
Settlements | 0 | | | |
Net unrealized gains (losses) on derivatives still held included in earnings | 0 | $ 2 | | |
Long- Term Liabilities | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - liabilities | 13 | | 7 | |
Level 1 | Current Assets | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - assets | 21 | | 21 | |
Level 1 | Current Assets | Equity Securities [Member] | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Restricted Investments, at Fair Value | 1 | | 0 | |
Level 1 | Long- Term Assets | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - assets | 4 | | 1 | |
Level 1 | Long- Term Assets | Equity Securities [Member] | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Restricted Investments, at Fair Value | 25 | | 22 | |
Level 1 | Current Liabilities | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - liabilities | 41 | | 19 | |
Level 1 | Long- Term Liabilities | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - liabilities | 5 | | 0 | |
Level 2 | Current Assets | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - assets | 37 | | 42 | |
Level 2 | Current Assets | Equity Securities [Member] | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Restricted Investments, at Fair Value | 1 | | 0 | |
Level 2 | Long- Term Assets | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - assets | 11 | | 13 | |
Level 2 | Long- Term Assets | Equity Securities [Member] | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Restricted Investments, at Fair Value | 0 | | 1 | |
Level 2 | Current Liabilities | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - liabilities | 32 | | 34 | |
Level 2 | Long- Term Liabilities | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - liabilities | 7 | | 6 | |
Level 3 | Current Assets | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - assets | 0 | | 0 | |
Level 3 | Current Assets | Equity Securities [Member] | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Restricted Investments, at Fair Value | 0 | | 0 | |
Level 3 | Long- Term Assets | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - assets | 2 | | 2 | |
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 | |
Level 3 | Current Liabilities | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - liabilities | 4 | | 3 | |
Level 3 | Long- Term Liabilities | Commodity derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Commodity derivatives - liabilities | $ 1 | | $ (1) | |