Summary of Significant Accounting Policies Cumulative Effect of the Adopted Accounting Pronouncements (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, Current	$ 2,164	$ 2,013	$ 1,969
Prepaid expenses and other current assets	1,323	1,204	1,040
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	395	367	250
Other Assets, Noncurrent	2,894	2,636	2,298
Accounts Payable, Current	345	438	933
Accrued Liabilities, Current	4,418	4,322	3,931
Other Liabilities, Current	1,519	748	792
Other liabilities	1,750	2,066	1,438
Retained earnings	25,086	22,547	22,364
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting principle		88	366
Adoption of intra-entity asset transfers standard
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting principle		186	$ (183)
Accounts Receivable, Net, Current		0
Prepaid expenses and other current assets		(17)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent		186
Other Assets, Noncurrent		(352)
Accounts Payable, Current		0
Accrued Liabilities, Current		0
Other Liabilities, Current		0
Other liabilities		0
Retained earnings		(183)
Impact of revenue standard | ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts Receivable, Net, Current	70	44
Prepaid expenses and other current assets	193	181
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	(83)	(69)
Other Assets, Noncurrent	791	690
Accounts Payable, Current	(618)	(495)
Accrued Liabilities, Current	549	391
Other Liabilities, Current	(61)	(44)
Other liabilities	664	628
Retained earnings	$ 437	$ 366