Intangible Assets

12 Months Ended

Dec. 31, 2023

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible Assets

7. Intangible Assets

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

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 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 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