Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions		12 Months Ended
	Mar. 31, 2019 country segment	Dec. 31, 2021 USD ($) segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of business segments (segment) | segment		2
Restricted cash		$ 303	$ 270
Investment income (loss)		61	(22)	$ 22
Gains on equity securities		5	27	10
Gain related to investments in private equity funds and other investments				12
Capitalized computer software costs		475	481
Accumulated amortization		1,700	1,600
Liability for errors and omission, current and noncurrent		434	639
Net uncollected premiums and claims and related payables		13,000	11,200
Lease liability		2,212
ROU asset		1,868	1,894
Accounts Payable and Accrued Liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Liabilities for errors and omissions, current		$ 79	271
Office Space | Lease Obligations | Lease Concentration Risk
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk (as a percent)		99.00%
Minimum | Customer Lists
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets, useful life		10 years
Minimum | Furniture and equipment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		3 years
Minimum | Building
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		30 years
Minimum | Software Development
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		3 years
Maximum | Customer Lists
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets, useful life		15 years
Maximum | Furniture and equipment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		10 years
Maximum | Building
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		40 years
Maximum | Software Development
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		10 years
JLT Transaction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of business segments (segment) | segment	3
Number of countries in which entity operates (country) | country	41
Risk and insurance services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, fiduciary assets		$ 15	$ 46	$ 105