Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	$ 176.8
Other payable	4.9
Recorded earn-out payable	51.3	$ 42.1
Total consideration	233.0
Maximum potential earn-out payable	98.9
Allocation of purchase price:
Cash and equivalents	4.5
Fiduciary cash	9.1
Fiduciary receivables	19.3
Other current assets	14.9
Fixed assets	0.1
Goodwill	154.2
Deferred income tax, net	13.6
Other assets	0.1
Total assets acquired	271.4
Fiduciary liabilities	(28.3)
Other current liabilities	(6.5)
Other long-term liabilities	(3.6)
Total liabilities assumed	(38.4)
Net assets acquired	233.0
Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets	55.3
Non-compete Agreements
Allocation of purchase price:
Intangible assets	0.3
Other Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	176.8
Other payable	4.9
Recorded earn-out payable	51.3
Total consideration	233.0
Maximum potential earn-out payable	98.9
Allocation of purchase price:
Cash and equivalents	4.5
Fiduciary cash	9.1
Fiduciary receivables	19.3
Other current assets	14.9
Fixed assets	0.1
Goodwill	154.2
Deferred income tax, net	13.6
Other assets	0.1
Total assets acquired	271.4
Fiduciary liabilities	(28.3)
Other current liabilities	(6.5)
Other long-term liabilities	(3.6)
Total liabilities assumed	(38.4)
Net assets acquired	233.0
Other Acquisitions | Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets	55.3
Other Acquisitions | Non-compete Agreements
Allocation of purchase price:
Intangible assets	$ 0.3