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Debt (Narrative) (Details) £ in Billions		1 Months Ended							3 Months Ended
Debt (Narrative) (Details) £ in Billions	Sep. 18, 2018 USD ($)	Mar. 31, 2020 USD ($)	Jan. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Apr. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Oct. 31, 2018	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Apr. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Apr. 01, 2019 USD ($)	Mar. 31, 2019 EUR (€)	Jan. 31, 2019 USD ($)	Oct. 01, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 18, 2018 GBP (£)
Debt Instrument [Line Items]
Commercial paper outstanding		$ 193,000,000											$ 0
$300 Million One-Year Term Loan Facility
Debt Instrument [Line Items]
Debt facilities, maximum borrowing capacity							$ 300,000,000			$ 300,000,000
$300 Million Three-Year Term Loan Facility
Debt Instrument [Line Items]
Long-term line of credit							$ 300,000,000			300,000,000
1.349% Senior Debt Obligations Due 2026
Debt Instrument [Line Items]
Interest rate		1.349%
1.979% Senior Debt Obligations Due 2030
Debt Instrument [Line Items]
Interest rate		1.979%
Commercial Paper
Debt Instrument [Line Items]
Short-term debt		$ 1,500,000,000
Effective interest rate		3.04%
Jardine Lloyd Thompson Group plc
Debt Instrument [Line Items]
Repayments of debt					$ 553,000,000	$ 450,000,000
Debt assumed in acquisition														$ 1,000,000,000
Payments for early extinguishment of debt									$ 32,000,000
Jardine Lloyd Thompson Group plc \| Bridge Loan
Debt Instrument [Line Items]
Derivative, notional amount \| £																			£ 5.2
Payments of debt issuance costs	$ 35,000,000
Amortization of debt issuance costs											$ 30,000,000
Interest expense, debt										$ 5,000,000
Term Loan Facility \| $500 Million One-Year Term Loan and $500 Million Two-Year Term Loan
Debt Instrument [Line Items]
Long-term line of credit		$ 1,000,000,000
Term Loan Facility \| $500 Million One-Year Term Loan Facility
Debt Instrument [Line Items]
Debt facilities, maximum borrowing capacity			$ 500,000,000
Term of debt			1 year
Term Loan Facility \| $500 Million Two-Year Term Loan Facility
Debt Instrument [Line Items]
Debt facilities, maximum borrowing capacity			$ 500,000,000
Term of debt			2 years
Term Loan Facility \| $300 Million One-Year Term Loan Facility
Debt Instrument [Line Items]
Term of debt							1 year
Term Loan Facility \| $300 Million Three-Year Term Loan Facility
Debt Instrument [Line Items]
Term of debt							3 years
Revolving Credit Facility \| Amended Revolving Credit Facility
Debt Instrument [Line Items]
Debt facilities, maximum borrowing capacity																	$ 1,800,000,000	$ 1,500,000,000
Term of debt								5 years
Long-term line of credit		1,000,000,000
Senior Notes
Debt Instrument [Line Items]
Repayments of debt		500,000,000		$ 300,000,000
Debt instrument face amount																$ 5,000,000,000
Senior Notes \| 1.349% Senior Debt Obligations Due 2026
Debt Instrument [Line Items]
Debt instrument face amount \| €															€ 550,000,000
Interest rate							1.349%			1.349%					1.349%
Senior Notes \| 1.979% Senior Debt Obligations Due 2030
Debt Instrument [Line Items]
Debt instrument face amount \| €															€ 550,000,000
Interest rate							1.979%			1.979%					1.979%
Senior Notes \| Senior Debt Obligations 4.375% Due 2029
Debt Instrument [Line Items]
Debt instrument face amount		$ 1,500,000,000					$ 250,000,000			$ 250,000,000						1,250,000,000
Interest rate		4.375%
Senior Notes \| Senior Debt Obligations 3.50% Due 2020
Debt Instrument [Line Items]
Debt instrument face amount																$ 700,000,000
Interest rate																3.50%
Senior Notes \| Senior Debt Obligations 3.875% Due 2024
Debt Instrument [Line Items]
Debt instrument face amount																$ 1,000,000,000
Interest rate																3.875%
Senior Notes \| Senior Debt Obligations 4.75% Due 2039
Debt Instrument [Line Items]
Debt instrument face amount																$ 500,000,000
Interest rate																4.75%
Senior Notes \| Senior Debt Obligations 4.90% Due 2049
Debt Instrument [Line Items]
Debt instrument face amount																$ 1,250,000,000
Interest rate																4.90%
Senior Notes \| Floating Rate Senior Notes Due 2021
Debt Instrument [Line Items]
Debt instrument face amount																$ 300,000,000
Senior Notes \| Jardine Lloyd Thompson Group plc
Debt Instrument [Line Items]
Debt instrument face amount \| €															€ 1,100,000,000
Subsequent Event \| April 2020 New Credit Facility
Debt Instrument [Line Items]
Debt facilities, maximum borrowing capacity												$ 1,000,000,000