Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2017

Goodwill and Intangible Assets Disclosure [Abstract]

Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations

The carrying amount of goodwill at December 31, 2017 and 2016 allocated by domestic and foreign operations is as follows (in millions):

	Brokerage		Risk Management		Corporate		Total
At December 31, 2017
United States	$	2,314.2	$	25.8	$	—	$	2,340.0
United Kingdom		738.5		7.2		—		745.7
Canada		375.3		—		—		375.3
Australia		416.6		—		—		416.6
New Zealand		209.3		9.6		—		218.9
Other foreign		98.4		—		3.0		101.4
Total goodwill - net	$	4,152.3	$	42.6	$	3.0	$	4,197.9
At December 31, 2016
United States	$	2,115.0	$	23.5	$	—	$	2,138.5
United Kingdom		662.2		4.3		—		666.5
Canada		292.2		—		—		292.2
Australia		382.7		—		—		382.7
New Zealand		205.0		0.3		—		205.3
Other foreign		79.8		—		2.8		82.6
Total goodwill – net	$	3,736.9	$	28.1	$	2.8	$	3,767.8

Changes in Carrying Amount of Goodwill

The changes in the carrying amount of goodwill for 2017 and 2016 are as follows (in millions):

	Brokerage			Risk Management			Corporate		Total
Balance as of January 1, 2016	$	3,635.6		$	27.3		$	—	$	3,662.9
Goodwill acquired during the year		222.6			—			2.8		225.4
Goodwill adjustments related to appraisals and other acquisition adjustments		1.8			1.6			—		3.4
Foreign currency translation adjustments during the year		(123.1	)		(0.8	)		—		(123.9	)
Balance as of December 31, 2016		3,736.9			28.1			2.8		3,767.8
Goodwill acquired during the year		290.4			14.1			—		304.5
Goodwill adjustments related to appraisals and other acquisition adjustments		14.7			—			—		14.7
Foreign currency translation adjustments during the year		110.3			0.4			0.2		110.9
Balance as of December 31, 2017	$	4,152.3		$	42.6		$	3.0	$	4,197.9

Major Classes of Amortizable Intangible Assets

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

	December 31,
	2017			2016
Expiration lists	$	3,055.9		$	2,757.6
Accumulated amortization - expiration lists		(1,422.1	)		(1,143.0	)
		1,633.8			1,614.6
Non-compete agreements		53.5			49.3
Accumulated amortization - non-compete agreements		(46.1	)		(42.1	)
		7.4			7.2
Trade names		25.9			24.0
Accumulated amortization - trade names		(22.5	)		(18.5	)
		3.4			5.5
Net amortizable assets	$	1,644.6		$	1,627.3

Estimated Aggregate Amortization Expense

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

2018	$	257.2
2019		243.3
2020		226.7
2021		203.4
2022		180.6
Total	$	1,111.2