Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Details) $ in Millions	Sep. 30, 2022 USD ($)
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	$ 102.0
Fiduciary cash	513.0
Fiduciary receivables	161.5
Other current assets	85.5
Fixed assets	25.7
Goodwill	1,922.2
Operating lease right-of-use assets	24.7
Other assets	14.0
Total assets acquired	3,511.1
Fiduciary liabilities	(674.5)
Other current liabilities	(76.9)
Deferred income tax, net	(156.7)
Operating lease liabilities	(24.7)
Other long term liabilities	(12.2)
Total liabilities assumed	(945.0)
Net assets acquired	2,566.1
Orchid Intermediate Holdings L P
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	3.2
Fiduciary cash	40.5
Fiduciary receivables	12.5
Other current assets	0.1
Fixed assets	1.8
Goodwill	400.5
Other assets	1.9
Total assets acquired	585.7
Fiduciary liabilities	(53.0)
Other current liabilities	(9.5)
Deferred income tax, net	(30.2)
Total liabilities assumed	(98.7)
Net assets acquired	487.0
GRP (Jersey) Holdco Limited (GRP)
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	80.3
Fiduciary cash	457.5
Fiduciary receivables	127.4
Other current assets	86.9
Fixed assets	17.9
Goodwill	1,365.1
Operating lease right-of-use assets	18.7
Other assets	8.8
Total assets acquired	2,653.3
Fiduciary liabilities	(584.9)
Other current liabilities	(62.3)
Deferred income tax, net	(122.5)
Operating lease liabilities	(18.7)
Other long term liabilities	(10.4)
Total liabilities assumed	(798.8)
Net assets acquired	1,854.5
First Insurance Solutions Ltd. (First)
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	2.4
Fiduciary cash	1.4
Other current assets	1.0
Goodwill	12.9
Total assets acquired	24.6
Fiduciary liabilities	(1.4)
Other current liabilities	(0.5)
Total liabilities assumed	(1.9)
Net assets acquired	22.7
BdB Holdings Limited (BdB)
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	15.8
Fiduciary cash	13.6
Fiduciary receivables	21.6
Other current assets	5.1
Fixed assets	0.6
Goodwill	53.7
Other assets	3.1
Total assets acquired	129.8
Fiduciary liabilities	(35.2)
Other current liabilities	(2.1)
Deferred income tax, net	(4.0)
Other long term liabilities	(1.8)
Total liabilities assumed	(43.1)
Net assets acquired	86.7
Smithwick & Mariners Insurance, Inc. (Smithwick)
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	0.2
Goodwill	20.5
Total assets acquired	27.5
Other current liabilities	(1.7)
Total liabilities assumed	(1.7)
Net assets acquired	25.8
VistaNational Insurance Group, Inc. (VistaNational)
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	1.4
Fixed assets	0.1
Goodwill	21.2
Total assets acquired	28.9
Other current liabilities	(0.3)
Total liabilities assumed	(0.3)
Net assets acquired	28.6
Other Acquisitions | Adjustments from Prior Year Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	0.3
Other current assets	(9.2)
Fixed assets	5.3
Goodwill	48.3
Other assets	0.2
Total assets acquired	61.3
Other current liabilities	(0.5)
Total liabilities assumed	(0.5)
Net assets acquired	60.8
Purchased Customer Accounts and Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	661.3
Purchased Customer Accounts and Other | Orchid Intermediate Holdings L P
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	119.2
Purchased Customer Accounts and Other | GRP (Jersey) Holdco Limited (GRP)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	490.7
Purchased Customer Accounts and Other | First Insurance Solutions Ltd. (First)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	6.9
Purchased Customer Accounts and Other | BdB Holdings Limited (BdB)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	16.0
Purchased Customer Accounts and Other | Smithwick & Mariners Insurance, Inc. (Smithwick)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	6.7
Purchased Customer Accounts and Other | VistaNational Insurance Group, Inc. (VistaNational)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	6.1
Purchased Customer Accounts and Other | Other Acquisitions | Adjustments from Prior Year Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	15.7
Non-compete Agreements
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	1.2
Non-compete Agreements | BdB Holdings Limited (BdB)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	0.3
Non-compete Agreements | Smithwick & Mariners Insurance, Inc. (Smithwick)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	0.1
Non-compete Agreements | VistaNational Insurance Group, Inc. (VistaNational)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	0.1
Non-compete Agreements | Other Acquisitions | Adjustments from Prior Year Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	$ 0.7