Fair Value Measurements and Financial Instruments - Carrying Value and Estimated Fair Value of Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Carrying Value
Debt Instrument [Line Items]
Carrying value of long-term debt	$ 6,656	$ 6,354
Fair Value, Inputs, Level 2
Debt Instrument [Line Items]
Estimated fair value of long-term debt	6,783	6,434
Long-term Debt
Debt Instrument [Line Items]
Carrying amount of hedged item	394	888
Cumulative hedging adjustment included in the carrying amount	$ 4	$ 10

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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
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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.

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Amount of liability hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (a)
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Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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