Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations

The carrying amount of goodwill at December 31, 2023 and 2022 allocated by domestic and foreign operations is as follows (in millions):

	Brokerage		Risk Management		Corporate		Total
At December 31, 2023
United States	$	6,304.5	$	74.8	$	—	$	6,379.3
United Kingdom		2,493.4		18.5		—		2,511.9
Canada		623.7		—		—		623.7
Australia		514.6		135.9		—		650.5
New Zealand		204.2		9.6		—		213.8
Other foreign		1,077.4		—		19.0		1,096.4
Total goodwill - net	$	11,217.8	$	238.8	$	19.0	$	11,475.6
At December 31, 2022
United States	$	5,065.7	$	74.8	$	—	$	5,140.5
United Kingdom		2,180.2		17.7		—		2,197.9
Canada		569.7		—		—		569.7
Australia		467.6		10.2		—		477.8
New Zealand		203.8		9.5		—		213.3
Other foreign		871.1		—		19.1		890.2
Total goodwill - net	$	9,358.1	$	112.2	$	19.1	$	9,489.4

Changes in Carrying Amount of Goodwill

The changes in the carrying amount of goodwill for 2023 and 2022 are as follows (in millions):

	Brokerage			Risk Management			Corporate			Total
Balance as of December 31, 2021	$	8,544.6		$	100.9		$	20.7		$	8,666.2
Goodwill acquired during the year		693.9			16.0			—			709.9
Goodwill adjustments related to appraisals and other acquisition    adjustments		428.3			(1.6	)		0.2			426.9
Foreign currency translation adjustments during the year		(308.7	)		(3.1	)		(1.8	)		(313.6	)
Balance as of December 31, 2022		9,358.1			112.2			19.1			9,489.4
Goodwill acquired during the year		1,667.4			121.8			—			1,789.2
Goodwill adjustments related to appraisals and other acquisition    adjustments		20.0			(0.1	)		—			19.9
Foreign currency translation adjustments during the year		172.3			4.9			(0.1	)		177.1
Balance as of December 31, 2023	$	11,217.8		$	238.8		$	19.0		$	11,475.6

Major Classes of Amortizable Intangible Assets

Major classes of amortizable intangible assets consist of the following (in millions):

	December 31,
	2023			2022
Expiration lists	$	8,222.8		$	6,472.3
Accumulated amortization - expiration lists		(3,733.2	)		(3,178.5	)
		4,489.6			3,293.8
Non-compete agreements		112.2			91.3
Accumulated amortization - non-compete agreements		(74.9	)		(67.5	)
		37.3			23.8
Trade names		171.8			108.5
Accumulated amortization - trade names		(65.4	)		(54.0	)
		106.4			54.5
Net amortizable assets	$	4,633.3		$	3,372.1

Estimated Aggregate Amortization Expense

Estimated aggregate amortization expense for each of the next five years is as follows (in millions):

2024	$	623.2
2025		580.7
2026		537.7
2027		500.5
2028		459.2
Thereafter		1,932.0
Total	$	4,633.3