Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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Debt and lines of credit (Details) (USD $)	12 Months Ended			1 Months Ended
Debt and lines of credit (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	May 31, 2013	Aug. 31, 2012
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity through March 2019	$ 2,000,000,000
Commercial paper	0us-gaap_CommercialPaper
Long-term debt, gross	4,625,000,000us-gaap_DebtInstrumentCarryingAmount	5,125,000,000us-gaap_DebtInstrumentCarryingAmount
Proceeds from issuance of long-term debt	498,000,000	986,000,000	1,492,000,000
Interest and debt expense	94,000,000us-gaap_InterestAndDebtExpense	95,000,000us-gaap_InterestAndDebtExpense	85,000,000us-gaap_InterestAndDebtExpense
Interest paid	102,000,000us-gaap_InterestPaid	102,000,000us-gaap_InterestPaid	97,000,000us-gaap_InterestPaid
Notes payable
Debt Instrument [Line Items]
Long-term debt, gross				500,000,000	1,000,000,000	1,500,000,000
Payments of debt issuance costs				3,000,000	6,000,000	7,000,000
Proceeds from issuance of long-term debt				498,000,000	986,000,000	1,492,000,000
Repayments of long-term debt	1,000,000,000	1,500,000,000
Notes payable \| Notes due 2017 at 0.875%
Debt Instrument [Line Items]
Long-term debt, gross	250,000,000	0		250,000,000
Notes payable \| Notes due 2021 at 2.75%
Debt Instrument [Line Items]
Long-term debt, gross	250,000,000	0		250,000,000
Notes payable \| Notes due 2018 at 1.00%
Debt Instrument [Line Items]
Long-term debt, gross	500,000,000	500,000,000			500,000,000
Notes payable \| Notes due 2023 at 2.25%
Debt Instrument [Line Items]
Long-term debt, gross	500,000,000	500,000,000			500,000,000
Notes payable \| Notes due 2015 at 0.45%
Debt Instrument [Line Items]
Long-term debt, gross	750,000,000	750,000,000				750,000,000
Notes payable \| Notes due 2019 at 1.65%
Debt Instrument [Line Items]
Long-term debt, gross	$ 750,000,000	$ 750,000,000				$ 750,000,000