Accounting Policies - Schedule of Changes in Accounting Policy (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable, net	$ 2,509	$ 2,028	$ 2,025
Prepaid Expenses and Other Current Assets	247	231	224
Deferred Income Taxes	2,039	2,009	2,008
Accounts Payable — Trade	2,850	2,814	2,807
Other Current Liabilities	898	1,033	1,026
Retained Earnings	6,084	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, net			2,025
Prepaid Expenses and Other Current Assets			224
Deferred Income Taxes			2,008
Accounts Payable — Trade			2,807
Other Current Liabilities			1,026
Retained Earnings			$ 6,044
Accounting Standards Update 2014-09 | Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable, net	2,506
Prepaid Expenses and Other Current Assets	236
Deferred Income Taxes	2,038
Accounts Payable — Trade	2,842
Other Current Liabilities	887
Retained Earnings	6,080
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable, net	3	3
Prepaid Expenses and Other Current Assets	11	7
Deferred Income Taxes	1	1
Accounts Payable — Trade	8	7
Other Current Liabilities	11	7
Retained Earnings	$ (4)	$ (3)