Commitments and Contingencies - Airport Projects (Details) - USD ($) $ in Millions		3 Months Ended
		Dec. 31, 2010	Jun. 30, 2018	Dec. 31, 2017
Airport Project [Line Items]
Assets constructed for others			$ 1,664	$ 1,543
Assets constructed for others, net			1,553	1,448
Construction obligation			1,613	1,390
Fort Lauderdale-Hollywood International Airport
Airport Project [Line Items]
Total Expected Cost Of Airport Project			333
Assets constructed for others	[1]		299	258
Assets constructed for others, net	[1],[2]		295	256
Construction obligation	[1],[3]		297	258
Los Angeles World Airport Terminal 1
Airport Project [Line Items]
Total Expected Cost Of Airport Project			526
Outstanding remaining guaranteed obligation			127
Total Expected Cost Of Airport Project Nonproprietary			510
Assets constructed for others	[1]		463	433
Assets constructed for others, net	[1],[2]		443	417
Construction obligation	[1],[3]		459	433
Los Angeles World Airport Terminal 1.5
Airport Project [Line Items]
Total Expected Cost Of Airport Project			479
Outstanding remaining guaranteed obligation			61
Assets constructed for others	[1]		54	31
Assets constructed for others, net	[1],[2]		54	31
Construction obligation	[1],[3]		54	31
Dallas Love Field Terminal
Airport Project [Line Items]
Assets constructed for others			544	543
Assets constructed for others, net	[2]		467	474
Construction obligation	[3]		509	516
Municipal bonds issued		$ 456
Municipal bonds principal remaining			424
Dallas Love Field Parking Garage
Airport Project [Line Items]
Assets constructed for others	[1]		178	152
Assets constructed for others, net	[1],[2]		178	152
Construction obligation	[1],[3]		178	152
Houston Hobby Airport
Airport Project [Line Items]
Airport Cost			150
Reimbursement for Airport Project			116
Assets constructed for others			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 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.