Commitments and Contingencies (Details) - USD ($) $ in Millions		3 Months Ended
		Sep. 30, 2017	Dec. 31, 2010	Dec. 31, 2016
Airport Project [Line Items]
Assets constructed for others		$ 1,460		$ 1,220
Assets constructed for others, net		1,373		1,156
Construction obligation		1,311		1,078
Fort Lauderdale-Hollywood International Airport
Airport Project [Line Items]
Total Expected Cost Of Airport Project		333
Assets constructed for others	[1]	245		132
Assets constructed for others, net	[1],[2]	244		132
Construction obligation	[1]	245		132
Los Angeles World Airport Terminal 1
Airport Project [Line Items]
Total Expected Cost Of Airport Project		526
Total Expected Cost Of Airport Project Nonproprietary		510
Remaining guaranteed obligation		299
Assets constructed for others	[1]	417		344
Assets constructed for others, net	[1],[2]	404		336
Construction obligation	[1]	417		344
Los Angeles World Airport Terminal 1.5
Airport Project [Line Items]
Total Expected Cost Of Airport Project		479
Remaining guaranteed obligation		0
Dallas Love Field Terminal
Airport Project [Line Items]
Assets constructed for others		540		538
Assets constructed for others, net	[2]	474		486
Construction obligation		517		522
Municipal bonds issued			$ 456
Municipal bonds principal remaining		432
Dallas Love Field Parking Garage
Airport Project [Line Items]
Assets constructed for others	[1]	132		80
Assets constructed for others, net	[1],[2]	132		80
Construction obligation	[1]	132		80
Houston Hobby Airport
Airport Project [Line Items]
Assets constructed for others	[3]	126		126
Assets constructed for others, net	[2],[3]	119		122
Construction obligation	[3]	$ 0		$ 0

[1] Projects still in progress.

[2] Net of accumulated depreciation.

[3] Project completed in 2015 at Houston William P. Hobby Airport ("HOU").