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Commitments and Contingencies - Airport Projects (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2010 | Sep. 30, 2018 | Dec. 31, 2017
Other Commitments [Line Items] |  |  |  |
ACFO |  |  | $ 1,718 | $ 1,543
Assets constructed for others, net |  |  | 1,598 | 1,448
Construction Obligation |  |  | 1,658 | 1,390
FLL Terminal |  |  |  |
Other Commitments [Line Items] |  |  |  |
Total Expected Cost Of Airport Project |  |  | 333 |
ACFO | [1] |  | 308 | 258
Assets constructed for others, net | [1],[2] |  | 302 | 256
Construction Obligation | [1],[3] |  | 304 | 258
LAX Terminal 1 |  |  |  |
Other Commitments [Line Items] |  |  |  |
Total Expected Cost Of Airport Project |  |  | 526 |
Outstanding remaining guaranteed obligation |  |  | 137 |
Total Expected Cost Of Airport Project Nonproprietary |  |  | 510 |
ACFO | [1] |  | 474 | 433
Assets constructed for others, net | [1],[2] |  | 451 | 417
Construction Obligation | [1],[3] |  | 467 | 433
LAX Terminal 1.5 |  |  |  |
Other Commitments [Line Items] |  |  |  |
Total Expected Cost Of Airport Project |  |  | 479 |
Outstanding remaining guaranteed obligation |  |  | 83 |
ACFO | [1] |  | 73 | 31
Assets constructed for others, net | [1],[2] |  | 73 | 31
Construction Obligation | [1],[3] |  | 73 | 31
LFMP Terminal |  |  |  |
Other Commitments [Line Items] |  |  |  |
ACFO |  |  | 545 | 543
Assets constructed for others, net | [2] |  | 464 | 474
Construction Obligation | [3] |  | 506 | 516
Municipal bonds issued |  | $ 456 |  |
Municipal bonds principal remaining |  |  | 424 |
LFMP Parking Garage |  |  |  |
Other Commitments [Line Items] |  |  |  |
ACFO | [1] |  | 192 | 152
Assets constructed for others, net | [1],[2] |  | 192 | 152
Construction Obligation | [1],[3] |  | 192 | 152
HOU International Terminal |  |  |  |
Other Commitments [Line Items] |  |  |  |
Airport Cost |  |  | 150 |
Reimbursement for Airport Project |  |  | 116 |
ACFO |  |  | 126 | 126
Assets constructed for others, net | [2] |  | 116 | 118
Construction Obligation | [3] |  | $ 116 | $ 0
[1] | Projects still in progress.
[2] | Net of accumulated depreciation.
[3] | Construction obligation will be reduced through future facility rent payments. These future rent payments are not fixed per the lease agreement, but are variable and fluctuate based on various market and other factors outside the control of the Company.

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