Segment and Related Information - Geographical (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 3,842	$ 3,822	$ 3,739	$ 3,511	$ 3,652	$ 3,716	$ 3,677	$ 3,440	$ 14,914	$ 14,485	$ 13,609
Property and equipment, net	11,942				11,559				11,942	11,559	10,950
US
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues									14,167	13,768	12,915
Property and equipment, net	11,044				10,591				11,044	10,591	10,040
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues									747	717	694
Property and equipment, net	$ 898				$ 968				$ 898	$ 968	$ 910