Summary of Significant Accounting Policies (Narrative) (Details)		12 Months Ended
	Mar. 31, 2019 USD ($) country segment	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2020 country	Jan. 01, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of business segments (segment) | segment		2
Restricted cash		$ 270,000,000
Investment (loss) income		(22,000,000)	$ 22,000,000	$ (12,000,000)
Gains on equity securities		(27,000,000)	10,000,000	54,000,000
Gain related to investments in private equity funds and other investments			12,000,000	17,000,000
Indefinite lived identified intangible assets		0	0
Capitalized computer software costs		481,000,000	496,000,000
Accumulated amortization		1,600,000,000	1,400,000,000
Liability for errors and omission, current and noncurrent		639,000,000	484,000,000
Net uncollected premiums and claims and related payables		11,200,000,000	8,900,000,000
Lease liability		2,266,000,000
ROU asset	$ 1,700,000,000	1,894,000,000	1,921,000,000
Accounts Payable and Accrued Liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Liabilities for errors and omissions, current		271,000,000	149,000,000
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liability						$ 1,900,000,000
ROU asset						1,700,000,000
Other liabilities						$ (200,000,000)
Alexander Forbes
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment (loss) income		$ (23,000,000)
Impairment charge				83,000,000
Minimum | Customer relationships
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 relationships
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				41
Risk and insurance services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, fiduciary assets		$ 46,000,000	105,000,000	65,000,000
Consulting Segment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, fiduciary assets		$ 1,000,000	$ 4,000,000	$ 3,000,000