Fair Value Measurements - Fair Value of Long-Term Fixed Rate and Variable Rate Debt Excluding Capital Leases (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	$ 5,973	$ 5,427
Notes payable and overdrafts	335	262
Fixed Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	3,604	3,616
Fair value — liability	3,541	3,786
Notes payable and overdrafts	184	110
Variable Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	2,369	1,811
Fair value — liability	2,353	1,811
Notes payable and overdrafts	$ 151	$ 152