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Contractual interest rate for funds borrowed, under the debt agreement.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Debt (Narrative) (Details)	1 Months Ended						3 Months Ended		12 Months Ended
Debt (Narrative) (Details)	Dec. 31, 2020 USD ($)	Apr. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jan. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Oct. 31, 2018	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	May 31, 2020 USD ($)	Apr. 01, 2019 USD ($)	Mar. 31, 2019 EUR (€)	Jan. 31, 2019 USD ($)	Oct. 01, 2018 USD ($)	Sep. 30, 2018 USD ($)
Debt Instrument [Line Items]
Short-term debt	$ 517,000,000								$ 517,000,000	$ 1,215,000,000
Commercial paper outstanding	0								0
Payments for early extinguishment of debt									0	585,000,000	$ 0
Repayments of principal in 2021	517,000,000								517,000,000
Repayments of principal in 2022	516,000,000								516,000,000
Repayments of principal in 2023	619,000,000								619,000,000
Repayments of principal in 2024	1,600,000,000								1,600,000,000
Repayments of principal in 2025	518,000,000								518,000,000
April 2020 New Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity		$ 1,000,000,000
Debt instrument, term		1 year
Other Debt Facilities
Debt Instrument [Line Items]
Revolving credit facility, amount outstanding	0								0	0
Debt instrument, unused borrowing capacity	573,000,000								573,000,000	598,000,000
Commercial Paper
Debt Instrument [Line Items]
Short-term debt	1,500,000,000								1,500,000,000
Term loan facility \| $500 Million One-Year Term Loan Facility
Debt Instrument [Line Items]
Repayments of debt	500,000,000						$ 500,000,000	$ 1,000,000,000
Revolving credit facility, borrowing capacity				$ 500,000,000
Debt instrument, term				1 year			1 year
Revolving credit facility, amount outstanding	$ 0								0
Term loan facility \| $500 Million Two-Year Term Loan Facility
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity				$ 500,000,000
Debt instrument, term	2 years			2 years
Revolving credit facility \| Amended Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity																$ 1,800,000,000	$ 1,500,000,000
Debt instrument, term						5 years
Revolving credit facility, amount outstanding	$ 0								$ 0
JLT Transaction
Debt Instrument [Line Items]
Indebtedness assumed													$ 1,000,000,000
Payments for early extinguishment of debt										32,000,000
Senior Notes
Debt Instrument [Line Items]
Repayments of debt	700,000,000		$ 500,000,000		$ 300,000,000
Debt instrument, amount															$ 5,000,000,000
Senior Notes \| 2.250% Senior Debt Obligations Due 2030
Debt Instrument [Line Items]
Debt instrument, amount												$ 750,000,000
Interest rate												2.25%
Senior Notes \| JLT Transaction
Debt Instrument [Line Items]
Debt instrument, amount										$ 6,500,000,000				€ 1,100,000,000
Floating Rate Notes
Debt Instrument [Line Items]
Repayments of debt	$ 300,000,000