Segment and Related Information - Summary of Net Operating Revenues by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Total operating revenues	$ 3,246	$ 3,360	$ 3,315	$ 3,040	$ 3,437	$ 3,602	$ 3,561	$ 3,396	$ 12,961	$ 13,996	$ 13,983
United States and Puerto Rico [Member]
Segment Reporting Information [Line Items]
Total operating revenues									12,196	13,064	13,054
Canada [Member]
Segment Reporting Information [Line Items]
Total operating revenues									$ 765	$ 932	$ 929