Segment and Related Information - Summary of Net Operating Revenues by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Total operating revenues	$ 3,437	$ 3,602	$ 3,561	$ 3,396	$ 3,500	$ 3,621	$ 3,526	$ 3,336	$ 13,996	$ 13,983	$ 13,649
United States and Puerto Rico [Member]
Segment Reporting Information [Line Items]
Total operating revenues									13,064	13,054	12,812
Canada [Member]
Segment Reporting Information [Line Items]
Total operating revenues									$ 932	$ 929	$ 837