| **Fair Value Measurements - Fair Values of Note Payable and Long-Term Debt (Detail) - USD ($) $ in Millions**                     | Sep. 30, 2020 | Dec. 31, 2019 |
|-----------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of note payable                                                                                                        | $ 123         | $ 39          |
| Total                                                                                                                             | $ 1,209       | $ 1,091       |
| Senior Debentures 6.900 % Due 2028                                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Interest rate                                                                                                                     | 6.90%         | 6.90%         |
| Fair value of senior debt                                                                                                         | $ 35          | $ 34          |
| Senior Debentures 6.920 % Due 2028                                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Interest rate                                                                                                                     | 6.92%         | 6.92%         |
| Fair value of senior debt                                                                                                         | $ 518         | $ 506         |
| Senior Notes 6.125 % Due 2034                                                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Interest rate                                                                                                                     | 6.125%        | 6.125%        |
| Fair value of senior debt                                                                                                         | $ 533         | $ 512         |
| Quoted prices in active markets for identical assets (Level 1)                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of note payable                                                                                                        | 0             | 0             |
| Total                                                                                                                             | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Senior Debentures 6.900 % Due 2028 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of senior debt                                                                                                         | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Senior Debentures 6.920 % Due 2028 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of senior debt                                                                                                         | 0             | 0             |
| Quoted prices in active markets for identical assets (Level 1) | Senior Notes 6.125 % Due 2034      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of senior debt                                                                                                         | 0             | 0             |
| Significant other observable inputs (Level 2)                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of note payable                                                                                                        | 123           | 39            |
| Total                                                                                                                             | 1,209         | 1,091         |
| Significant other observable inputs (Level 2) | Senior Debentures 6.900 % Due 2028                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of senior debt                                                                                                         | 35            | 34            |
| Significant other observable inputs (Level 2) | Senior Debentures 6.920 % Due 2028                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of senior debt                                                                                                         | 518           | 506           |
| Significant other observable inputs (Level 2) | Senior Notes 6.125 % Due 2034                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of senior debt                                                                                                         | 533           | 512           |
| Significant unobservable inputs (Level 3)                                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of note payable                                                                                                        | 0             | 0             |
| Total                                                                                                                             | 0             | 0             |
| Significant unobservable inputs (Level 3) | Senior Debentures 6.900 % Due 2028                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of senior debt                                                                                                         | 0             | 0             |
| Significant unobservable inputs (Level 3) | Senior Debentures 6.920 % Due 2028                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of senior debt                                                                                                         | 0             | 0             |
| Significant unobservable inputs (Level 3) | Senior Notes 6.125 % Due 2034                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis**                                               |          |          |
| Fair value of senior debt                                                                                                         | $ 0           | $ 0           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611
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| - Definition
Contractual interest rate for funds borrowed, under the debt agreement.
+ References
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| - Definition
The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.
+ References
No definition available.
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| - Definition
Fair value portion of notes payable.
+ References
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