Summary of Significant Accounting Policies (Narrative) (Details)		12 Months Ended
	Mar. 31, 2019 USD ($) country segment	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of business segments (segment) | segment		2
Net uncollected premiums and claims and related payables		$ 8,900,000,000	$ 7,300,000,000
Restricted cash		197,000,000
Investment gain		22,000,000		$ 15,000,000
Investment loss			12,000,000
Gains on equity securities			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		496,000,000	435,000,000
Accumulated amortization		1,400,000,000	1,300,000,000
Lease liability		2,268,000,000
ROU asset	$ 1,700,000,000	1,921,000,000
Retained earnings		15,199,000,000	14,347,000,000
Cumulative effect of accounting standard			(14,000,000)
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		105,000,000	65,000,000	39,000,000
Consulting Segment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, fiduciary assets		$ 4,000,000	3,000,000	$ 4,000,000
Alexander Forbes
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impairment charge			$ 83,000,000
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 | 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
Customer relationships | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets, useful life		10 years
Customer relationships | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets, useful life		15 years
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liability					$ 1,900,000,000
ROU asset					1,700,000,000
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings						$ 364,000,000
Restatement Adjustment | ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other liabilities					$ (200,000,000)
Retained Earnings | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting standard						14,000,000
Retained Earnings | ASU 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting standard						$ (14,000,000)