Segment and Related Information - Summary of Net Operating Revenues by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total operating revenues	$ 3,460	$ 3,548	$ 3,425	$ 3,176	$ 3,246	$ 3,360	$ 3,315	$ 3,040	$ 13,609	$ 12,961	$ 13,996
United States and Puerto Rico [Member]
Segment Reporting Information [Line Items]
Total operating revenues									12,915	12,196	13,064
Canada [Member]
Segment Reporting Information [Line Items]
Total operating revenues									$ 694	$ 765	$ 932