Fair Value Measurements (Fair Value of Note Payable and Long-Term Debt) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of note payable	$ 24	$ 20
Total	1,040	976
6.900% Senior Debentures, Due 2028
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	$ 34	$ 33
Interest rate	6.90%	6.90%
Debt instrument, year of maturity	2028	2028
6.920% Senior Debentures, Due 2028
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	$ 505	$ 488
Interest rate	6.92%	6.92%
Debt instrument, year of maturity	2028	2028
6.125% Senior Notes, Due 2034
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	$ 477	$ 435
Interest rate	6.125%	6.125%
Debt instrument, year of maturity	2034	2034
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of note payable	$ 0	$ 0
Total	0	0
Level 1 \| 6.900% Senior Debentures, Due 2028
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	0	0
Level 1 \| 6.920% Senior Debentures, Due 2028
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	0	0
Level 1 \| 6.125% Senior Notes, Due 2034
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	0	0
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of note payable	24	20
Total	1,040	976
Level 2 \| 6.900% Senior Debentures, Due 2028
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	34	33
Level 2 \| 6.920% Senior Debentures, Due 2028
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	505	488
Level 2 \| 6.125% Senior Notes, Due 2034
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	477	435
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of note payable	0	0
Total	0	0
Level 3 \| 6.900% Senior Debentures, Due 2028
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	0	0
Level 3 \| 6.920% Senior Debentures, Due 2028
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	0	0
Level 3 \| 6.125% Senior Notes, Due 2034
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Fair value of senior debt	$ 0	$ 0

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13476-108611

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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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- 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

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- Definition

Fair value portion of notes payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20,22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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