Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 68	$ 54	$ 55
Acquired	14	6	0
Charged to Costs and Expenses	45	40	24
Deductions	34	32	25
Balance at End of Period	93	68	54
Reserve for obsolescence and shrinkage
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	7	3	4
Acquired	1	2	0
Charged to Costs and Expenses	26	20	17
Deductions	29	18	18
Balance at End of Period	5	7	3
Self-insurance reserve
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	100	94	90
Acquired	5	6	0
Charged to Costs and Expenses	144	122	108
Deductions	143	122	104
Balance at End of Period	$ 106	$ 100	$ 94