Quarterly Financial Information (Unaudited) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Revenues	$ 2,306,000,000	$ 2,116,000,000	$ 1,891,000,000	$ 1,734,000,000	$ 1,922,000,000	$ 1,766,000,000	$ 1,597,000,000	$ 1,356,000,000	$ 8,047,000,000	$ 6,641,000,000	$ 5,762,000,000
Gross profit	998,000,000	938,000,000	782,000,000	646,000,000	827,000,000	773,000,000	655,000,000	514,000,000	3,364,000,000	2,769,000,000	2,403,000,000
Operating income	563,000,000	578,000,000	470,000,000	340,000,000	462,000,000	448,000,000	340,000,000	257,000,000	1,951,000,000	1,507,000,000	1,415,000,000
Net income	$ 310,000,000	$ 333,000,000	$ 270,000,000	$ 183,000,000	$ 897,000,000	$ 199,000,000	$ 141,000,000	$ 109,000,000	$ 1,096,000,000	$ 1,346,000,000	$ 566,000,000
Earnings per share - basic (in dollars per share)	$ 3.84	$ 4.05	$ 3.22	$ 2.18	$ 10.60	$ 2.36	$ 1.67	$ 1.29	$ 13.26	$ 15.91	$ 6.49
Earnings per share - diluted (in dollars per share)	$ 3.80	4.01	3.20	2.15	$ 10.45	2.33	1.65	1.27	$ 13.12	$ 15.73	$ 6.45
Effect of Fourth Quarter Events [Line Items]
Benefit of Tax Cuts and Jobs Act of 2017					$ 689,000,000				$ (6,000,000)	$ 689,000,000
Effect on Tax Cuts and Jobs Act of 2017 on diluted earnings per share (in dollars per share)					$ 8.03					$ 8.05
Merger related costs	$ 22,000,000				$ 18,000,000				36,000,000	$ 50,000,000	$ 0
Restructuring charge	$ 16,000,000				22,000,000				31,000,000	50,000,000	14,000,000
Loss on extinguishment of debt									$ 0	$ 54,000,000	$ 101,000,000
Increase in stock compensation, net					$ 11,000,000
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Merger related costs (in dollars per share)	$ (0.21)	(0.09)	(0.02)	(0.01)	$ (0.13)	(0.12)	(0.09)	(0.02)	$ (0.32)	$ (0.36)
Merger related intangible asset amortization (in dollars per share)	(0.58)	(0.42)	(0.37)	(0.39)	(0.32)	(0.27)	(0.30)	(0.28)	(1.76)	(1.15)
Impact on depreciation related to acquired fleet and property and equipment (in dollars per share)	0	(0.02)	(0.08)	(0.09)	(0.01)	(0.07)	0.03	0	(0.19)	(0.05)
Impact of the fair value mark-up of acquired fleet (in dollars per share)	(0.11)	(0.11)	(0.15)	(0.21)	(0.23)	(0.17)	(0.13)	(0.06)	(0.59)	(0.59)
Restructuring charge (in dollars per share)	$ (0.15)	$ (0.09)	$ (0.03)	$ (0.02)	(0.15)	(0.07)	(0.14)	0	$ (0.28)	(0.36)
Asset impairment charge (in dollars per share)					0	0	0	0		(0.01)
Loss on extinguishment of debt securities and amendment of ABL facility (in dollars per share)					$ (0.08)	$ (0.22)	$ (0.09)	$ 0		$ (0.39)
Senior notes | 7 5/8 percent Senior Notes
Effect of Fourth Quarter Events [Line Items]
Redemption of principal amount of debt					$ 225,000,000					$ 225,000,000
Stated interest rate	7.625%								7.625%
Loss on extinguishment of debt					$ 11,000,000