Segment and Related Information - Geographical (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues	$ 3,846	$ 3,967	$ 3,946	$ 3,696	$ 3,842	$ 3,822	$ 3,739	$ 3,511	$ 15,455	$ 14,914	$ 14,485
Property and equipment, net	12,893				11,942				12,893	11,942	11,559
US
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues									14,701	14,167	13,768
Property and equipment, net	11,941				11,044				11,941	11,044	10,591
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues									754	747	717
Property and equipment, net	$ 952				$ 898				$ 952	$ 898	$ 968