Fair Value Measurements - Fair Value of Long-Term Fixed Rate and Variable Rate Debt Excluding Capital Leases (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	$ 5,886	$ 5,427
Notes payable and overdrafts	332	262
Fixed Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	3,625	3,616
Fair value — liability	3,651	3,786
Notes payable and overdrafts	176	110
Variable Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	2,261	1,811
Fair value — liability	2,248	1,811
Notes payable and overdrafts	$ 156	$ 152