Accounting Policies - Schedule of Changes in Accounting Policy (Details) - USD ($) $ in Millions	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable, net	$ 2,388	$ 2,028	$ 2,025
Prepaid Expenses and Other Current Assets	265	231	224
Deferred Income Taxes	2,035	2,009	2,008
Accounts Payable — Trade	2,880	2,814	2,807
Other Current Liabilities	821	1,033	1,026
Retained Earnings	6,208	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	2,380
Prepaid Expenses and Other Current Assets	255
Deferred Income Taxes	2,036
Accounts Payable — Trade	2,875
Other Current Liabilities	811
Retained Earnings	6,206
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	8	3
Prepaid Expenses and Other Current Assets	10	7
Deferred Income Taxes	(1)	1
Accounts Payable — Trade	5	7
Other Current Liabilities	10	7
Retained Earnings	$ 2	$ (3)