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Fair Value Measurements - Measured on Recurring Basis (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | $ 283 | $ 444
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | (57) | (805)
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Assets |  |  |
Treasury bills |  | 1,491 | 1,345
Fuel derivatives: |  |  |
Other available for sale securities |  | 107 | 83
Total assets |  | 2,731 | 2,772
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
Total assets |  | 649 | 658
Fuel derivatives: |  |  |
Interest rate derivatives (see Note 10) |  | (22) | (35)
Total Liabilities |  | (22) | (145)
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Assets |  |  |
Treasury bills |  | 0 | 0
Fuel derivatives: |  |  |
Total assets |  | 283 | 402
Fuel derivatives: |  |  |
Interest rate derivatives (see Note 10) |  | 0 | 0
Total Liabilities |  | (35) | (660)
Cash Equivalents | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Assets |  |  |
Cash equivalents | [2] | 1,133 | 1,344
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 |  | 350 | 325
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 |  | 12 | 11
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 |  | 287 | 280
Certificates of deposit | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Assets |  |  |
Investments |  | 0 | 0
Swap | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Asset | [3] |  | 0
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Liability | [3] |  | 0
Swap | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) |  |  |
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Asset | [3] |  | 42
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Liability | [3] |  | (110)
Swap | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Asset | [3] |  | 0
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Liability | [3] |  | 0
Options Held | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) |  |  |
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Asset | [3] |  | 0
Derivative Asset, Fair Value, Gross Asset | [4] | 0 | 0
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Liability | [4] | 0 | 0
Derivative Liability, Fair Value, Gross Liability | [3] |  | 0
Options Held | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) |  |  |
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Asset | [3] |  | 0
Derivative Asset, Fair Value, Gross Asset | [4] | 0 | 0
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Liability | [4] | 0 | 0
Derivative Liability, Fair Value, Gross Liability | [3] |  | 0
Options Held | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) |  |  |
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Asset | [3] |  | 163
Derivative Asset, Fair Value, Gross Asset | [4] | 283 | 239
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Liability | [4] | (35) | (96)
Derivative Liability, Fair Value, Gross Liability | [3] |  | (564)
Period end date | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Treasury bills |  | 1,491 | 1,345
Fuel derivatives: |  |  |
Other available for sale securities |  | 107 | 83
Total assets |  | 3,663 | 3,832
Fuel derivatives: |  |  |
Interest rate derivatives (see Note 10) |  | (22) | (35)
Total Liabilities |  | (57) | (805)
Period end date | Cash Equivalents | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Cash equivalents | [2] | 1,133 | 1,344
Period end date | Commercial paper | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Cash equivalents |  | 350 | 325
Period end date | Certificates of deposit | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Cash equivalents |  | 12 | 11
Period end date | Certificates of deposit | Fair Value, Measurements, Recurring |  |  |
Assets |  |  |
Investments |  | 287 | 280
Period end date | Swap | Fair Value, Measurements, Recurring |  |  |
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Asset | [3] |  | 42
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Liability | [3] |  | (110)
Period end date | Options Held | Fair Value, Measurements, Recurring |  |  |
Fuel derivatives: |  |  |
Derivative Liability, Fair Value, Gross Asset | [3] |  | 163
Derivative Asset, Fair Value, Gross Asset | [4] | 283 | 239
Fuel derivatives: |  |  |
Derivative Asset, Fair Value, Gross Liability | [4] | $ (35) | (96)
Derivative Liability, Fair Value, Gross Liability | [3] |  | $ (564)
[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 a net liability. See Note 10.
[4] | In the Consolidated Balance Sheet amounts are presented as a net asset. See Note 10.

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