Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	$ 695.1
Common stock issued	18.3	$ 14.7	$ 9.9
Other payable	12.2
Recorded earn-out payable	67.2	$ 73.3	$ 75.7
Total consideration	792.8
Maximum potential earn-out payable	142.5
Allocation of purchase price:
Cash and equivalents	21.8
Fiduciary cash	42.6
Fiduciary receivables	67.6
Other current assets	71.8
Goodwill	608.7
Purchased customer accounts and other	167.4
Deferred income tax, net	4.9
Other assets	12.2
Total assets acquired	997.0
Fiduciary liabilities	(110.3)
Other current liabilities	(52.3)
Deferred income tax, net	(16.6)
Other long-term liabilities	(25.0)
Total liabilities assumed	(204.2)
Net assets acquired	792.8
Kentro Capital Limited | Retail
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	402.5
Common stock issued	18.3
Other payable	2.2
Total consideration	423.0
Allocation of purchase price:
Cash and equivalents	8.5
Fiduciary cash	27.3
Fiduciary receivables	38.8
Other current assets	37.8
Goodwill	358.7
Purchased customer accounts and other	66.0
Other assets	6.1
Total assets acquired	543.2
Fiduciary liabilities	66.1
Other current liabilities	22.6
Deferred income tax, net	16.6
Other long-term liabilities	14.9
Total liabilities assumed	120.2
Net assets acquired	423.0
Berkeley Insurance Group Limited | Retail
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	48.0
Other payable	1.3
Recorded earn-out payable	15.4
Total consideration	64.7
Maximum potential earn-out payable	21.0
Allocation of purchase price:
Cash and equivalents	0.4
Fiduciary cash	2.8
Fiduciary receivables	2.2
Other current assets	10.9
Goodwill	43.3
Purchased customer accounts and other	22.0
Other assets	0.5
Total assets acquired	82.1
Fiduciary liabilities	5.0
Other current liabilities	12.2
Other long-term liabilities	0.2
Total liabilities assumed	17.4
Net assets acquired	64.7
Other Acquisitions | Various
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	244.6
Other payable	8.7
Recorded earn-out payable	51.8
Total consideration	305.1
Maximum potential earn-out payable	121.5
Allocation of purchase price:
Cash and equivalents	12.9
Fiduciary cash	12.5
Fiduciary receivables	26.6
Other current assets	23.1
Goodwill	206.7
Purchased customer accounts and other	79.4
Deferred income tax, net	4.9
Other assets	5.6
Total assets acquired	371.7
Fiduciary liabilities	39.2
Other current liabilities	17.5
Other long-term liabilities	9.9
Total liabilities assumed	66.6
Net assets acquired	$ 305.1