Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 54	$ 55	$ 43
Acquired	6	0	0
Charged to Costs and Expenses	40	24	32
Deductions	32	25	20
Balance at End of Period	68	54	55
Reserve for obsolescence and shrinkage
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	3	4	3
Acquired	2	0	0
Charged to Costs and Expenses	20	17	18
Deductions	18	18	17
Balance at End of Period	7	3	4
Self-insurance reserve
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	94	90	92
Acquired	6	0	0
Charged to Costs and Expenses	122	108	110
Deductions	122	104	112
Balance at End of Period	$ 100	$ 94	$ 90