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Debt (Narrative) (Details) £ in Billions		1 Months Ended							3 Months Ended		12 Months Ended
Debt (Narrative) (Details) £ in Billions	Sep. 18, 2018 USD ($)	Jan. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Aug. 31, 2019	Jun. 30, 2019 USD ($)	Apr. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Oct. 31, 2018 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Apr. 01, 2019 USD ($)	Mar. 31, 2019 EUR (€)	Jan. 31, 2019 USD ($)	Oct. 01, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 18, 2018 GBP (£)	Mar. 31, 2018 USD ($)
Debt Instrument [Line Items]
Short-term debt											$ 1,215,000,000	$ 314,000,000
Commercial paper outstanding											0
Payments for early extinguishment of debt											585,000,000	0	$ 0
Repayments of principal in 2020											1,200,000,000
Repayments of principal in 2021											815,000,000
Repayments of principal in 2022											515,000,000
Repayments of principal in 2023											617,000,000
Repayments of principal in 2024											$ 1,600,000,000
$300 Million One-Year Term Loan Facility
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity							$ 300,000,000			$ 300,000,000
$300 Million Three-Year Term Loan Facility
Debt Instrument [Line Items]
Revolving credit facility, amount outstanding							$ 300,000,000			$ 300,000,000
Senior Notes Due 2026
Debt Instrument [Line Items]
Interest rate											1.349%
Senior Notes Due 2030
Debt Instrument [Line Items]
Interest rate											1.979%
Senior Notes Due 2048
Debt Instrument [Line Items]
Debt instrument, amount																				$ 600,000,000
Interest rate											4.20%									4.20%
Amended Revolving Credit Facility November 24, 2015
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity																		$ 1,500,000,000
Revolving credit facility, amount outstanding											$ 0
Amended Revolving Credit Facility March 28, 2014
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity																	$ 1,800,000,000
Other Debt Facilities
Debt Instrument [Line Items]
Revolving credit facility, amount outstanding											0
Debt instrument, unused borrowing capacity											598,000,000	$ 594,000,000
Commercial Paper
Debt Instrument [Line Items]
Short-term debt											1,500,000,000
Senior Notes
Debt Instrument [Line Items]
Debt instrument, amount																$ 5,000,000,000
Debt maturity			$ 300,000,000					$ 250,000,000
Senior Notes \| Senior Notes Due 2026
Debt Instrument [Line Items]
Debt instrument, amount \| €															€ 550,000,000
Interest rate							1.349%			1.349%					1.349%
Senior Notes \| Senior Notes Due 2030
Debt Instrument [Line Items]
Debt instrument, amount \| €															€ 550,000,000
Interest rate							1.979%			1.979%					1.979%
Senior Notes \| Senior Notes Due 2029
Debt Instrument [Line Items]
Debt instrument, amount							$ 250,000,000			$ 250,000,000	$ 1,500,000,000					$ 1,250,000,000
Interest rate							4.375%			4.375%	4.375%				4.375%	4.375%
Senior Notes \| Senior Notes Due 2020
Debt Instrument [Line Items]
Debt instrument, amount																$ 700,000,000
Interest rate																3.50%
Senior Notes \| Senior Notes Due 2024
Debt Instrument [Line Items]
Debt instrument, amount																$ 1,000,000,000.0
Interest rate																3.875%
Senior Notes \| Senior Notes Due 2039
Debt Instrument [Line Items]
Debt instrument, amount																$ 500,000,000
Interest rate																4.75%
Senior Notes \| Senior Notes Due 2049
Debt Instrument [Line Items]
Debt instrument, amount																$ 1,250,000,000
Interest rate																4.90%
Senior Notes \| Floating Rate Senior Notes Due 2021
Debt Instrument [Line Items]
Debt instrument, amount																$ 300,000,000
JLT Transaction
Debt Instrument [Line Items]
Debt maturity					$ 553,000,000	$ 450,000,000
Indebtedness assumed											$ 1,044,000,000			$ 1,000,000,000
Payments for early extinguishment of debt									$ 32,000,000
JLT Transaction \| 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
JLT Transaction \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, amount \| €															€ 1,100,000,000
Term loan facility \| $500 Million One-Year Term Loan Facility
Debt Instrument [Line Items]
Debt instrument, term							1 year
Term loan facility \| $300 Million One-Year Term Loan Facility
Debt Instrument [Line Items]
Debt instrument, term							1 year
Term loan facility \| $300 Million Three-Year Term Loan Facility
Debt Instrument [Line Items]
Debt instrument, term				3 years			3 years
Term loan facility \| Loan Facilities Closed March 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 instrument, term								5 years
Subsequent Event \| Term loan facility
Debt Instrument [Line Items]
Revolving credit facility, amount outstanding		$ 0
Subsequent Event \| Term loan facility \| $500 Million One-Year Term Loan Facility
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity		$ 500,000,000
Debt instrument, term		1 year
Subsequent Event \| 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