Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail) $ in Millions	Dec. 31, 2023 USD ($)
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	$ 146.0
Fiduciary assets	190.4
Other current assets	177.2
Fixed assets	112.1
Noncurrent assets	85.6
Goodwill	1,789.2
Expiration lists	1,721.0
Non-compete agreements	30.5
Trade names	64.3
Total assets acquired	4,316.3
Fiduciary liabilities	190.4
Current liabilities	149.8
Noncurrent liabilities	233.5
Total liabilities assumed	573.7
Total net assets acquired	3,742.6
FIR [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	13.0
Fiduciary assets	13.8
Other current assets	1.4
Fixed assets	0.8
Noncurrent assets	8.6
Goodwill	63.8
Expiration lists	25.9
Non-compete agreements	0.1
Trade names	0.4
Total assets acquired	127.8
Fiduciary liabilities	13.8
Current liabilities	3.9
Noncurrent liabilities	12.3
Total liabilities assumed	30.0
Total net assets acquired	97.8
BCHR [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	25.8
Other current assets	60.9
Fixed assets	36.8
Noncurrent assets	35.0
Goodwill	366.8
Expiration lists	207.6
Trade names	2.6
Total assets acquired	735.5
Current liabilities	50.4
Noncurrent liabilities	64.3
Total liabilities assumed	114.7
Total net assets acquired	620.8
BFH [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	0.6
Fiduciary assets	3.7
Other current assets	1.4
Noncurrent assets	0.2
Goodwill	50.6
Expiration lists	22.7
Non-compete agreements	0.6
Total assets acquired	79.8
Fiduciary liabilities	3.7
Current liabilities	0.6
Noncurrent liabilities	6.2
Total liabilities assumed	10.5
Total net assets acquired	69.3
TRH [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	5.3
Other current assets	8.3
Noncurrent assets	0.2
Goodwill	61.9
Expiration lists	7.5
Trade names	0.4
Total assets acquired	83.6
Current liabilities	2.6
Noncurrent liabilities	2.0
Total liabilities assumed	4.6
Total net assets acquired	79.0
IKB [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	2.0
Fiduciary assets	0.5
Other current assets	2.6
Fixed assets	0.2
Noncurrent assets	0.5
Goodwill	49.8
Expiration lists	22.3
Non-compete agreements	0.5
Total assets acquired	78.4
Fiduciary liabilities	0.5
Current liabilities	1.7
Noncurrent liabilities	6.3
Total liabilities assumed	8.5
Total net assets acquired	69.9
RHP [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Fiduciary assets	3.0
Other current assets	1.5
Noncurrent assets	0.3
Goodwill	43.7
Expiration lists	43.5
Trade names	0.2
Total assets acquired	92.2
Fiduciary liabilities	3.0
Current liabilities	1.3
Noncurrent liabilities	11.5
Total liabilities assumed	15.8
Total net assets acquired	76.4
CLM [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Fiduciary assets	15.8
Other current assets	3.3
Fixed assets	1.5
Noncurrent assets	4.0
Goodwill	59.9
Expiration lists	34.5
Non-compete agreements	0.6
Total assets acquired	119.6
Fiduciary liabilities	15.8
Current liabilities	3.8
Noncurrent liabilities	15.7
Total liabilities assumed	35.3
Total net assets acquired	84.3
EIG [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	0.5
Fiduciary assets	22.9
Other current assets	20.4
Fixed assets	0.1
Noncurrent assets	1.5
Goodwill	191.1
Expiration lists	322.0
Non-compete agreements	1.0
Total assets acquired	559.5
Fiduciary liabilities	22.9
Current liabilities	20.4
Noncurrent liabilities	1.1
Total liabilities assumed	44.4
Total net assets acquired	515.1
CDI [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	58.7
Fiduciary assets	42.3
Other current assets	40.7
Fixed assets	12.3
Noncurrent assets	16.0
Goodwill	314.7
Expiration lists	499.5
Non-compete agreements	1.6
Total assets acquired	985.8
Fiduciary liabilities	42.3
Current liabilities	38.6
Noncurrent liabilities	18.9
Total liabilities assumed	99.8
Total net assets acquired	886.0
MPM [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	5.0
Other current assets	5.5
Fixed assets	59.6
Noncurrent assets	4.8
Goodwill	121.8
Expiration lists	88.0
Non-compete agreements	14.7
Trade names	58.7
Total assets acquired	358.1
Current liabilities	4.8
Noncurrent liabilities	51.7
Total liabilities assumed	56.5
Total net assets acquired	301.6
Forty-one Other Acquisitions [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash and cash equivalents	35.1
Fiduciary assets	88.4
Other current assets	31.2
Fixed assets	0.8
Noncurrent assets	14.5
Goodwill	465.1
Expiration lists	447.5
Non-compete agreements	11.4
Trade names	2.0
Total assets acquired	1,096.0
Fiduciary liabilities	88.4
Current liabilities	21.7
Noncurrent liabilities	43.5
Total liabilities assumed	153.6
Total net assets acquired	$ 942.4