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Debt (Narrative) (Details) £ in Billions	1 Months Ended						3 Months Ended		9 Months Ended		12 Months Ended
Debt (Narrative) (Details) £ in Billions	Sep. 30, 2019 USD ($)	Aug. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Apr. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Oct. 31, 2018	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Apr. 01, 2019 USD ($)	Mar. 31, 2019 EUR (€)	Jan. 31, 2019 USD ($)	Oct. 01, 2018 USD ($)	Sep. 18, 2018 GBP (£)	Mar. 31, 2018 USD ($)
Debt Instrument [Line Items]
Commercial paper outstanding	$ 325,000,000								$ 325,000,000		$ 0
Payments for early extinguishment of debt									585,000,000	$ 0
Financing Program
Debt Instrument [Line Items]
Debt financing program	$ 1,500,000,000								$ 1,500,000,000
1.349% Senior Debt Obligations Due 2026
Debt Instrument [Line Items]
Interest rate	1.349%								1.349%
1.979% Senior Debt Obligations Due 2030
Debt Instrument [Line Items]
Interest rate	1.979%								1.979%
Loan Facility One
Debt Instrument [Line Items]
Debt facilities, maximum borrowing capacity					$ 300,000,000			$ 300,000,000
Loan Facility Two
Debt Instrument [Line Items]
Revolving credit facility, amount outstanding					$ 300,000,000			300,000,000
4.20% Senior Debt Obligations Due 2048
Debt Instrument [Line Items]
Debt instrument face amount																	$ 600,000,000
Interest rate	4.20%								4.20%								4.20%
Commercial Paper
Debt Instrument [Line Items]
Effective interest rate	2.29%								2.29%
Jardine Lloyd Thompson Group plc
Debt Instrument [Line Items]
Repayments of debt			$ 553,000,000	$ 450,000,000
Payments for early extinguishment of debt							$ 32,000,000
Debt assumed in acquisition	$ 1,044,000,000								$ 1,044,000,000			$ 1,000,000,000
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
Senior Notes
Debt Instrument [Line Items]
Debt instrument face amount														$ 5,000,000,000
Repayments of debt	300,000,000
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 4.375% Due 2029
Debt Instrument [Line Items]
Debt instrument face amount	1,500,000,000								1,500,000,000					$ 1,250,000,000
Interest rate														4.375%
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 \| 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 2019
Debt Instrument [Line Items]
Debt instrument face amount					$ 250,000,000			$ 250,000,000
Senior Notes \| Jardine Lloyd Thompson Group plc
Debt Instrument [Line Items]
Debt instrument face amount \| €													€ 1,100,000,000
Term Loan Facility \| Loan Facility One
Debt Instrument [Line Items]
Term of debt					1 year
Term Loan Facility \| Loan Facility Two
Debt Instrument [Line Items]
Term of debt		3 years			3 years
Term Loan Facility \| Loan Facilities Closed March 2019
Debt Instrument [Line Items]
Revolving credit facility, amount outstanding	$ 300,000,000								$ 300,000,000
Term of debt									1 year
Debt, average borrowing rate (percentage)	3.05%								3.05%
Term Loan Facility \| Loan Facilities Canceled August 2019
Debt Instrument [Line Items]
Revolving credit facility, amount outstanding		$ 300,000,000
Revolving Credit Facility \| Amended Revolving Credit Facility
Debt Instrument [Line Items]
Debt facilities, maximum borrowing capacity										$ 1,500,000,000					$ 1,800,000,000
Revolving credit facility, amount outstanding	$ 0								$ 0
Term of debt						5 years