Fair Value Measurements - Fair Value of Long-Term Fixed Rate and Variable Rate Debt Excluding Capital Leases (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	$ 5,766	$ 5,316
Notes Payable and Overdrafts	495	410
Fixed Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	3,402	3,609
Fair value — liability	3,342	3,443
Notes Payable and Overdrafts	261	230
Variable Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	2,364	1,707
Fair value — liability	2,339	1,689
Notes Payable and Overdrafts	$ 234	$ 180