Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Nov. 30, 2019	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
Selected Quarterly Financial Information [Abstract]
Revenues		$ 2,456	$ 2,488	$ 2,290	$ 2,117	$ 2,306	$ 2,116	$ 1,891	$ 1,734	$ 9,351	$ 8,047	$ 6,641
Gross profit		965	1,033	911	761	998	938	782	646	3,670	3,364	2,769
Operating income		599	656	529	368	563	578	470	340	2,152	1,951	1,507
Net income		$ 338	$ 391	$ 270	$ 175	$ 310	$ 333	$ 270	$ 183	$ 1,174	$ 1,096	$ 1,346
Earnings per share - basic (in dollars per share)		$ 4.51	$ 5.10	$ 3.45	$ 2.21	$ 3.84	$ 4.05	$ 3.22	$ 2.18	$ 15.18	$ 13.26	$ 15.91
Earnings per share - diluted (in dollars per share)		4.49	5.08	3.44	2.19	$ 3.80	4.01	3.20	2.15	$ 15.11	$ 13.12	$ 15.73
Effect of Fourth Quarter Events [Line Items]
Merger related costs						$ 22				$ 1	$ 36	$ 50
Restructuring charge						$ 16				18	31	50
Loss on extinguishment of debt										$ 61	$ 0	$ 54
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Merger related costs (in dollars per share)		0	0	0	(0.01)	$ (0.21)	(0.09)	(0.02)	(0.01)	$ (0.01)	$ (0.32)
Merger related intangible asset amortization (in dollars per share)		(0.60)	(0.63)	(0.64)	(0.64)	(0.58)	(0.42)	(0.37)	(0.39)	(2.48)	(1.76)
Impact on depreciation related to acquired fleet and property and equipment (in dollars per share)		(0.05)	(0.07)	(0.12)	(0.14)	0	(0.02)	(0.08)	(0.09)	(0.39)	(0.19)
Impact of the fair value mark-up of acquired fleet (in dollars per share)		(0.16)	(0.14)	(0.15)	(0.25)	(0.11)	(0.11)	(0.15)	(0.21)	(0.72)	(0.59)
Restructuring charge (in dollars per share)		(0.03)	(0.02)	(0.06)	(0.07)	$ (0.15)	$ (0.09)	$ (0.03)	$ (0.02)	(0.18)	$ (0.28)
Asset impairment charge (in dollars per share)		0.01	(0.02)	(0.03)	(0.01)					(0.05)
Loss on extinguishment of debt securities and amendment of ABL facility (in dollars per share)		$ (0.28)	$ 0	$ (0.30)	$ 0					$ (0.58)
Senior notes | 4 7/8 percent Senior Notes due 2028
Effect of Fourth Quarter Events [Line Items]
Loss on extinguishment of debt	$ 29
Stated interest rate		4.875%								4.875%
Senior notes | 4 5/8 percent senior notes
Effect of Fourth Quarter Events [Line Items]
Stated interest rate	3.875%	3.875%								3.875%
Senior notes | 4 5/8 percent Senior Notes due 2025
Effect of Fourth Quarter Events [Line Items]
Stated interest rate		4.625%								4.625%