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Fair Value Measurements - Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Assets |  |  |  |
Treasury bills | $ 1,291 |  | $ 1,582 |
Interest rate derivatives (see Note 3) | 0 |  |  |
Fuel derivatives: |  |  |  |
Other available for sale securities | 172 |  | 127 |
Total assets | 3,579 |  | 3,101 |
Liabilities |  |  |  |
Interest rate derivatives (see Note 3) | 0 |  | 0 |
Significant other observable inputs (Level 2) |  |  |  |
Assets |  |  |  |
Treasury bills | 0 |  | 0 |
Fuel derivatives: |  |  |  |
Other available for sale securities | 0 |  | 0 |
Total assets | 579 |  | 715 |
Liabilities |  |  |  |
Interest rate derivatives (see Note 3) | (37) |  | (14) |
Significant unobservable inputs (Level 3) |  |  |  |
Assets |  |  |  |
Treasury bills | 0 |  | 0 |
Interest rate derivatives (see Note 3) | 0 |  |  |
Fuel derivatives: |  |  |  |
Other available for sale securities | 0 |  | 0 |
Total assets | 187 |  | 138 |
Liabilities |  |  |  |
Interest rate derivatives (see Note 3) | 0 |  | 0 |
Options Held | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Fuel derivatives: |  |  |  |
Derivative Asset, Fair Value, Gross Asset | 0 | [1] | 0 | [2]
Options Held | Significant other observable inputs (Level 2) |  |  |  |
Fuel derivatives: |  |  |  |
Derivative Asset, Fair Value, Gross Asset | 0 | [1] | 0 | [2]
Options Held | Significant unobservable inputs (Level 3) |  |  |  |
Fuel derivatives: |  |  |  |
Derivative Asset, Fair Value, Gross Asset | 187 | [1] | 138 | [2]
Cash Equivalents | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Assets |  |  |  |
Cash equivalents | 2,116 | [3] | 1,392 | [4]
Cash Equivalents | Significant other observable inputs (Level 2) |  |  |  |
Assets |  |  |  |
Cash equivalents | 0 | [3] | 0 | [4]
Cash Equivalents | Significant unobservable inputs (Level 3) |  |  |  |
Assets |  |  |  |
Cash equivalents | 0 | [3] | 0 | [4]
Commercial Paper | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Assets |  |  |  |
Cash equivalents | 0 |  | 0 |
Commercial Paper | Significant other observable inputs (Level 2) |  |  |  |
Assets |  |  |  |
Cash equivalents | 295 |  | 454 |
Commercial Paper | Significant unobservable inputs (Level 3) |  |  |  |
Assets |  |  |  |
Cash equivalents | 0 |  | 0 |
Certificates of Deposit | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Assets |  |  |  |
Cash equivalents | 0 |  | 0 |
Investments | 0 |  | 0 |
Certificates of Deposit | Significant other observable inputs (Level 2) |  |  |  |
Assets |  |  |  |
Cash equivalents | 10 |  | 8 |
Investments | 224 |  | 228 |
Certificates of Deposit | Significant unobservable inputs (Level 3) |  |  |  |
Assets |  |  |  |
Cash equivalents | 0 |  | 0 |
Investments | 0 |  | 0 |
Time Deposits | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Assets |  |  |  |
Cash equivalents | 0 |  | 0 |
Time Deposits | Significant other observable inputs (Level 2) |  |  |  |
Assets |  |  |  |
Cash equivalents | 25 |  | 25 |
Time Deposits | Significant unobservable inputs (Level 3) |  |  |  |
Assets |  |  |  |
Cash equivalents | 0 |  | 0 |
Estimate of Fair Value Measurement |  |  |  |
Assets |  |  |  |
Treasury bills | 1,291 |  | 1,582 |
Fuel derivatives: |  |  |  |
Other available for sale securities | 172 |  | 127 |
Total assets | 4,345 |  | 3,954 |
Liabilities |  |  |  |
Interest rate derivatives (see Note 3) | (37) |  | (14) |
Estimate of Fair Value Measurement | Options Held |  |  |  |
Fuel derivatives: |  |  |  |
Derivative Asset, Fair Value, Gross Asset | 187 | [1] | 138 | [2]
Estimate of Fair Value Measurement | Cash Equivalents |  |  |  |
Assets |  |  |  |
Cash equivalents | 2,116 | [3] | 1,392 | [4]
Estimate of Fair Value Measurement | Commercial Paper |  |  |  |
Assets |  |  |  |
Cash equivalents | 295 |  | 454 |
Estimate of Fair Value Measurement | Certificates of Deposit |  |  |  |
Assets |  |  |  |
Cash equivalents | 10 |  | 8 |
Investments | 224 |  | 228 |
Estimate of Fair Value Measurement | Time Deposits |  |  |  |
Assets |  |  |  |
Cash equivalents | $ 25 |  | $ 25 |
[1] | In the unaudited Condensed Consolidated Balance Sheet amounts are presented as an asset. See Note 3.
[2] | In the unaudited Condensed Consolidated Balance Sheet amounts are presented as a net asset. See Note 3.
[3] | Cash equivalents are primarily composed of money market investments.
[4] | Cash equivalents are primarily composed of money market investments.

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