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.