Fair Value Measurements - Fair Value of Long-Term Fixed Rate and Variable Rate Debt Excluding Capital Leases (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	$ 6,038	$ 5,427
Notes payable and overdrafts	445	262
Fixed Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	3,601	3,616
Fair value — liability	3,551	3,786
Notes payable and overdrafts	245	110
Variable Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	2,437	1,811
Fair value — liability	2,421	1,811
Notes payable and overdrafts	$ 200	$ 152