Segment and Related Information - Geographical (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	$ 3,652	$ 3,716	$ 3,677	$ 3,440	$ 3,460	$ 3,548	$ 3,425	$ 3,176	$ 14,485	$ 13,609	$ 12,961
Property and equipment, net	11,559				10,950				11,559	10,950	10,665
US
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									13,768	12,915	12,196
Property and equipment, net	10,591				10,040				10,591	10,040	9,778
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues									717	694	765
Property and equipment, net	$ 968				$ 910				$ 968	$ 910	$ 887