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Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Assets					$ 131,506		$ 95,131
Liabilities					94,884	$ 57,449	57,436
Cash and cash equivalents and restricted cash
Cash and cash equivalents					41,587		1,696
Total cash, cash equivalents and restricted cash in the statement of cash flows	$ 1,900	$ 1,900	$ 3,520		41,827		1,900
Condensed Consolidated Statements of Cash Flows
Acquisitions (net of cash acquired) and other investments		(656)	(461)
Net cash used in investing activities		(1,386)	(1,567)
Net decrease in cash, cash equivalents and restricted cash		39,927	(845)
Cash, cash equivalents and restricted cash at the beginning of the period	1,900	1,900	3,520
Cash, cash equivalents and restricted cash at the end of the period		41,827	2,675
Adjustments Early Adoption \| ASU 2018-02 \| Adjustments
Condensed Consolidated Statements of Operations
Amounts reclassified from accumulated other comprehensive income (loss)	(7)
ASU 2016-18
Cash and cash equivalents and restricted cash
Cash and cash equivalents					41,587		1,696
Total cash, cash equivalents and restricted cash in the statement of cash flows	1,900	1,900			41,827		1,900
Condensed Consolidated Statements of Cash Flows
Cash, cash equivalents and restricted cash at the beginning of the period	$ 1,900	1,900
Cash, cash equivalents and restricted cash at the end of the period		$ 41,827
ASU 2016-18 \| Previously Reported
Cash and cash equivalents and restricted cash
Total cash, cash equivalents and restricted cash in the statement of cash flows			3,371
Condensed Consolidated Statements of Cash Flows
Acquisitions (net of cash acquired) and other investments			(502)
Net cash used in investing activities			(1,608)
Net decrease in cash, cash equivalents and restricted cash			(886)
Cash, cash equivalents and restricted cash at the beginning of the period			3,371
Cash, cash equivalents and restricted cash at the end of the period			2,485
ASU 2016-18 \| Adjustments
Cash and cash equivalents and restricted cash
Total cash, cash equivalents and restricted cash in the statement of cash flows			149
Condensed Consolidated Statements of Cash Flows
Acquisitions (net of cash acquired) and other investments			41
Net cash used in investing activities			41
Net decrease in cash, cash equivalents and restricted cash			41
Cash, cash equivalents and restricted cash at the beginning of the period			149
Cash, cash equivalents and restricted cash at the end of the period			$ 190
ASU 2016-18 \| Other Current Assets
Cash and cash equivalents and restricted cash
Restricted Cash and Cash Equivalents					11		14
ASU 2016-18 \| Other Long-Term Assets
Cash and cash equivalents and restricted cash
Restricted Cash and Cash Equivalents					$ 229		$ 190
ASU 2016-02 \| Minimum \| Forecast
New Accounting Pronouncements or Change in Accounting Principle
Assets				$ 19,000
Liabilities				19,000
ASU 2016-02 \| Maximum \| Forecast
New Accounting Pronouncements or Change in Accounting Principle
Assets				21,000
Liabilities				$ 21,000