Accounting Policies - Cumulative Effect of New Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable	$ 2,030	$ 2,028	$ 2,025
Prepaid Expenses and Other Current Assets	238	231	224
Deferred Income Taxes — Asset	1,847	2,009	2,008
Accounts Payable-Trade	2,920	2,814	2,807
Other Current Liabilities	737	1,033	1,026
Retained Earnings	6,597	6,041	6,044
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable			2,025
Prepaid Expenses and Other Current Assets			224
Deferred Income Taxes — Asset			2,008
Accounts Payable-Trade			2,807
Other Current Liabilities			1,026
Retained Earnings			$ 6,044
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable	2,018
Prepaid Expenses and Other Current Assets	228
Deferred Income Taxes — Asset	1,848
Accounts Payable-Trade	2,911
Other Current Liabilities	727
Retained Earnings	6,595
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable	12	3
Prepaid Expenses and Other Current Assets	10	7
Deferred Income Taxes — Asset	(1)	1
Accounts Payable-Trade	9	7
Other Current Liabilities	10	7
Retained Earnings	$ 2	$ (3)