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Loans Payable, Long-Term Debt and Other Commitments - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
	Nov. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Loans payable and current portion of long-term debt		$ 5,308,000,000	$ 3,057,000,000
Long-term debt, current maturities			3,000,000,000
Long-term debt		19,806,000,000	21,353,000,000
Debt extinguishment amount	$ 810,000,000
Repurchased face amount	$ 585,000,000
Loss on extinguishment of debt			191,000,000
Long-term debt, maturities, repayments of principal in 2019		0
Long-term debt, maturities, repayments of principal in 2020		1,900,000,000
Long-term debt, maturities, repayments of principal in 2021		2,300,000,000
Long-term debt, maturities, repayments of principal in 2022		2,200,000,000
Long-term debt, maturities, repayments of principal in 2023		1,700,000,000
Available borrowing capacity under credit facility		6,000,000,000.0
Rental expense under operating leases net of sublease income		322,000,000	327,000,000	$ 292,000,000
Minimum aggregate rental commitments under noncancellable leases in 2019		188,000,000
Minimum aggregate rental commitments under noncancellable leases in 2020		198,000,000
Minimum aggregate rental commitments under noncancellable leases in 2021		150,000,000
Minimum aggregate rental commitments under noncancellable leases in 2022		134,000,000
Minimum aggregate rental commitments under noncancellable leases in 2023		84,000,000
Minimum aggregate rental commitments under noncancellable leases thereafter		$ 243,000,000
5.00% notes due 2019 | Interest Rate Swap
Debt Instrument [Line Items]
Stated interest rate (as percent)		5.00%
Repurchased face amount		$ 1,250,000,000.00
Other Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt		$ 223,000,000	$ 300,000,000
Effective interest rate (as percent)		2.27%	1.42%
6.30% debentures due 2026
Debt Instrument [Line Items]
Long-term debt		$ 135,000,000	$ 135,000,000
Stated interest rate (as percent)		6.30%
Notes Subject To Repayment At Option Of Holder
Debt Instrument [Line Items]
Loans payable and current portion of long-term debt		$ 149,000,000	$ 73,000,000
Commercial paper
Debt Instrument [Line Items]
Loans payable and current portion of long-term debt		$ 5,100,000,000
Weighted-average interest rate of commercial paper (as percent)		2.09%	0.85%

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