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Fair Value Measurements - Measured on Recurring Basis (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Assets |  |  |
Time Deposits |  | $ 25 |
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 138 | $ 283
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | (14) | (57)
Quoted prices in active markets for identical assets (Level 1) |  |  |
Assets |  |  |
Time Deposits |  | 0 |
Significant other observable inputs (Level 2) |  |  |
Assets |  |  |
Time Deposits |  | 25 |
Significant unobservable inputs (Level 3) |  |  |
Assets |  |  |
Time Deposits |  | 0 |
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Assets |  |  |
Treasury bills |  | 1,582 | 1,491
Fuel derivatives: |  |  |
Other available for sale securities |  | 127 | 107
Assets, Fair Value Disclosure |  | 3,101 | 2,731
Fuel derivatives: |  |  |
Interest rate derivatives (see Note 10) |  | 0 | 0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) |  |  |
Assets |  |  |
Treasury bills |  | 0 | 0
Fuel derivatives: |  |  |
Other available for sale securities |  | 0 | 0
Assets, Fair Value Disclosure |  | 715 | 649
Fuel derivatives: |  |  |
Interest rate derivatives (see Note 10) |  | (14) | (22)
Total Liabilities |  |  | (22)
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Assets |  |  |
Treasury bills |  | 0 | 0
Fuel derivatives: |  |  |
Assets, Fair Value Disclosure |  | 138 | 283
Fuel derivatives: |  |  |
Interest rate derivatives (see Note 10) |  | 0 | 0
Total Liabilities |  |  | (35)
Cash Equivalents | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Assets |  |  |
Cash equivalents | [2] | 1,392 | 1,133
Cash Equivalents | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) |  |  |
Assets |  |  |
Cash equivalents | [2] | 0 | 0
Cash Equivalents | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Assets |  |  |
Cash equivalents | [2] | 0 | 0
Commercial paper | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Assets |  |  |
Cash equivalents |  | 0 | 0
Commercial paper | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) |  |  |
Assets |  |  |
Cash equivalents |  | 454 | 350
Commercial paper | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Assets |  |  |
Cash equivalents |  | 0 | 0
Certificates of deposit | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Assets |  |  |
Cash equivalents |  | 0 | 0
Certificates of deposit | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) |  |  |
Assets |  |  |
Cash equivalents |  | 8 | 12
Certificates of deposit | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Assets |  |  |
Cash equivalents |  | 0 | 0
Certificates of deposit | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Assets |  |  |
Investments |  | 0 | 0
Certificates of deposit | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) |  |  |
Assets |  |  |
Investments |  | 228 | 287
Certificates of deposit | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Assets |  |  |
Investments |  | 0 | 0
Options Held | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Asset | [3] | 0 | 0
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Liability | [3] |  | 0
Options Held | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) |  |  |
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Asset | [3] | 0 | 0
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Liability | [3] |  | 0
Options Held | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Asset | [3] | 138 | 283
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Liability | [3] |  | (35)
Period end date | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Treasury bills |  | 1,582 | 1,491
Fuel derivatives: |  |  |
Other available for sale securities |  | 127 | 107
Assets, Fair Value Disclosure |  | 3,954 | 3,663
Fuel derivatives: |  |  |
Interest rate derivatives (see Note 10) |  | (14) | (22)
Total Liabilities |  |  | (57)
Period end date | Cash Equivalents | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Cash equivalents | [2] | 1,392 | 1,133
Period end date | Commercial paper | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Cash equivalents |  | 454 | 350
Period end date | Certificates of deposit | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Cash equivalents |  | 8 | 12
Period end date | Certificates of deposit | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Investments |  | 228 | 287
Period end date | Options Held | Fair Value, Measurements, Recurring |  |  |
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Asset | [3] | $ 138 | 283
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Liability | [3] |  | $ (35)
[1] | Represents the position of each trade before consideration of offsetting positions with each counterparty and does not include the impact of cash collateral deposits provided to or received from counterparties. See discussion of credit risk and collateral following in this Note.
[2] | Cash equivalents are primarily composed of money market investments.
[3] | In the Consolidated Balance Sheet amounts are presented as an asset. See Note 10.

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