Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	$ 103.7	$ 57.8	$ 27.7
Fiduciary cash	513.0
Fiduciary receivables	176.0
Other current assets	95.1	77.0	22.0
Fixed assets	15.5	1.9	9.4
Goodwill	1,994.5	350.0	648.1
Operating lease right-of-use assets	24.7
Other assets	11.3	3.4	3.7
Total assets acquired	3,605.0	646.3	951.8
Fiduciary liabilities	(689.0)
Other current liabilities	(82.5)	(88.2)	(32.3)
Deferred income tax, net	(156.7)	(8.7)
Operating lease liabilities	(24.7)
Other liabilities	(14.1)		(26.3)
Total liabilities assumed	(967.0)	(96.9)	(58.6)
Net assets acquired	2,638.0	549.4	893.2
Orchid
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	411.9
Operating lease right-of-use assets	6.0
Other assets	1.9
Total assets acquired	585.7
Fiduciary liabilities	(53.0)
Other current liabilities	(9.5)
Deferred income tax, net	(30.2)
Operating lease liabilities	(6.0)
Total liabilities assumed	(98.7)
Net assets acquired	487.0
GRP
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	80.3
Fiduciary cash	457.5
Fiduciary receivables	141.9
Other current assets	86.9
Fixed assets	12.7
Goodwill	1,366.5
Operating lease right-of-use assets	18.7
Other assets	8.8
Total assets acquired	2,670.1
Fiduciary liabilities	(599.4)
Other current liabilities	(64.6)
Deferred income tax, net	(122.5)
Operating lease liabilities	(18.7)
Other liabilities	(10.4)
Total liabilities assumed	(815.6)
Net assets acquired	1,854.5
First
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	2.4
Fiduciary cash	1.4
Other current assets	0.9
Goodwill	14.5
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
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	15.8
Fiduciary cash	13.6
Fiduciary receivables	21.6
Other current assets	8.8
Fixed assets	0.6
Goodwill	56.0
Total assets acquired	130.7
Fiduciary liabilities	(35.2)
Other current liabilities	(3.3)
Deferred income tax, net	(4.0)
Other liabilities	(1.8)
Total liabilities assumed	(44.3)
Net assets acquired	86.4
Smithwick
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	0.8
Goodwill	19.7
Total assets acquired	27.6
Other current liabilities	(1.8)
Total liabilities assumed	(1.8)
Net assets acquired	25.8
VistaNational
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	1.7
Fixed assets	0.1
Goodwill	20.9
Total assets acquired	28.9
Other current liabilities	(0.3)
Total liabilities assumed	(0.3)
Net assets acquired	28.6
Taylor
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	1.0
Other current assets	3.7
Fixed assets	0.2
Goodwill	34.7
Total assets acquired	48.6
Other current liabilities	(1.6)
Other liabilities	(1.9)
Total liabilities assumed	(3.5)
Net assets acquired	45.1
O'Leary
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash		45.4
Other current assets		43.5
Fixed assets		0.5
Goodwill		84.6
Other assets		0.1
Total assets acquired		215.4
Other current liabilities		(72.6)
Deferred income tax, net		(5.1)
Total liabilities assumed		(77.7)
Net assets acquired		137.7
Piper
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		2.4
Goodwill		40.0
Total assets acquired		54.7
Net assets acquired		54.7
Berkshire
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		1.6
Goodwill		27.1
Other assets		0.5
Total assets acquired		41.5
Net assets acquired		41.5
AGIS
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		13.7
Goodwill		8.7
Total assets acquired		36.1
Net assets acquired		36.1
Winston
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash		5.0
Other current assets		7.5
Fixed assets		1.2
Goodwill		74.5
Total assets acquired		114.1
Other current liabilities		(5.5)
Total liabilities assumed		(5.5)
Net assets acquired		108.6
Remedy
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash		6.7
Other current assets		1.8
Fixed assets		0.1
Goodwill		33.3
Other assets		1.3
Total assets acquired		57.4
Other current liabilities		(5.2)
Deferred income tax, net		(3.6)
Total liabilities assumed		(8.8)
Net assets acquired		48.6
Heacock
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		0.8
Goodwill		13.9
Other assets		0.4
Total assets acquired		20.9
Other current liabilities		(0.3)
Total liabilities assumed		(0.3)
Net assets acquired		20.6
CIA
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill		17.3
Other assets		0.3
Total assets acquired		22.4
Other current liabilities		(0.4)
Total liabilities assumed		(0.4)
Net assets acquired		22.0
Rainmaker
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill		15.0
Total assets acquired		20.8
Net assets acquired		20.8
HARCO
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		0.7
Fixed assets		0.1
Goodwill		20.6
Other assets		0.5
Total assets acquired		30.1
Other current liabilities		(0.4)
Total liabilities assumed		(0.4)
Net assets acquired		29.7
Special Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			2.5
Fixed assets			0.3
Goodwill			63.1
Total assets acquired			80.3
Other current liabilities			(0.3)
Total liabilities assumed			(0.3)
Net assets acquired			80.0
Texas Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			0.5
Goodwill			9.0
Total assets acquired			12.7
Other current liabilities			(1.7)
Total liabilities assumed			(1.7)
Net assets acquired			11.0
Colonial
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			1.3
Fixed assets			0.1
Goodwill			27.9
Total assets acquired			38.5
Other current liabilities			(1.4)
Total liabilities assumed			(1.4)
Net assets acquired			37.1
RLA
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill			53.6
Total assets acquired			66.4
Other current liabilities			(11.4)
Total liabilities assumed			(11.4)
Net assets acquired			55.0
Sterling
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			0.6
Goodwill			17.3
Total assets acquired			23.9
Other current liabilities			(0.2)
Total liabilities assumed			(0.2)
Net assets acquired			23.7
LP
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			3.2
Fixed assets			1.9
Goodwill			100.0
Total assets acquired			149.9
Other liabilities			(0.2)
Total liabilities assumed			(0.2)
Net assets acquired			149.7
First
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			0.3
Goodwill			9.5
Total assets acquired			14.9
Net assets acquired			14.9
Buiten
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			2.6
Fixed assets			0.1
Goodwill			33.6
Total assets acquired			47.7
Other current liabilities			(0.9)
Total liabilities assumed			(0.9)
Net assets acquired			46.8
Amity
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			0.6
Fixed assets			0.1
Goodwill			15.5
Total assets acquired			21.8
Other current liabilities			(2.9)
Total liabilities assumed			(2.9)
Net assets acquired			18.9
Neal
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			2.3
Fixed assets			0.1
Goodwill			28.9
Other assets			0.3
Total assets acquired			44.8
Other current liabilities			(3.0)
Total liabilities assumed			(3.0)
Net assets acquired			41.8
Brook Stone
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			0.5
Fixed assets			0.1
Goodwill			8.6
Other assets			0.3
Total assets acquired			13.2
Other current liabilities			(0.1)
Total liabilities assumed			(0.1)
Net assets acquired			13.1
VAS
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash			27.7
Other current assets			5.5
Fixed assets			0.1
Goodwill			100.8
Total assets acquired			182.4
Other current liabilities			(3.7)
Other liabilities			(26.1)
Total liabilities assumed			(29.8)
Net assets acquired			152.6
Bright
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			0.4
Goodwill			12.2
Total assets acquired			17.7
Other current liabilities			(0.1)
Total liabilities assumed			(0.1)
Net assets acquired			17.6
J.E. Brown
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill			31.5
Total assets acquired			41.0
Other current liabilities			(0.7)
Total liabilities assumed			(0.7)
Net assets acquired			40.3
Cover Hound
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			0.4
Fixed assets			6.4
Goodwill			19.5
Total assets acquired			30.0
Other current liabilities			(1.8)
Total liabilities assumed			(1.8)
Net assets acquired			28.2
MAJ
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			0.4
Goodwill			13.0
Total assets acquired			21.4
Net assets acquired			21.4
South & Western
Fair Value Of Assets And Liabilities Statement [Line Items]
Fixed assets			0.2
Goodwill			63.1
Total assets acquired			81.8
Other current liabilities			(3.6)
Total liabilities assumed			(3.6)
Net assets acquired			78.2
Berry
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill			29.7
Total assets acquired			39.4
Other current liabilities			(0.4)
Total liabilities assumed			(0.4)
Net assets acquired			39.0
Other Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash		0.7
Other current assets		5.0	0.9
Goodwill		15.0	11.3
Other assets		0.3	3.1
Total assets acquired		32.9	24.0
Other current liabilities		(3.8)	(0.1)
Total liabilities assumed		(3.8)	(0.1)
Net assets acquired		29.1	23.9
Adjustments from Prior Year Acquisitions | Other Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	1.0
Other current assets	(7.8)
Fixed assets	0.1
Goodwill	70.3
Other assets	0.3
Total assets acquired	88.8
Other current liabilities	(0.9)
Total liabilities assumed	(0.9)
Net assets acquired	87.9
Purchased Customer Accounts
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		153.7	240.0
Purchased Customer Accounts | O'Leary
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		40.5
Purchased Customer Accounts | Piper
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		12.3
Purchased Customer Accounts | Berkshire
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		12.3
Purchased Customer Accounts | AGIS
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		13.6
Purchased Customer Accounts | Winston
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		25.0
Purchased Customer Accounts | Remedy
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		13.7
Purchased Customer Accounts | Heacock
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		5.8
Purchased Customer Accounts | CIA
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		4.8
Purchased Customer Accounts | Rainmaker
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		5.8
Purchased Customer Accounts | HARCO
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		8.2
Purchased Customer Accounts | Special Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			14.3
Purchased Customer Accounts | Texas Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			3.2
Purchased Customer Accounts | Colonial
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			9.2
Purchased Customer Accounts | RLA
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			12.3
Purchased Customer Accounts | Sterling
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			6.0
Purchased Customer Accounts | LP
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			44.8
Purchased Customer Accounts | First
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			5.1
Purchased Customer Accounts | Buiten
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			11.3
Purchased Customer Accounts | Amity
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			5.6
Purchased Customer Accounts | Neal
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			13.2
Purchased Customer Accounts | Brook Stone
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			3.7
Purchased Customer Accounts | VAS
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			48.2
Purchased Customer Accounts | Bright
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			5.1
Purchased Customer Accounts | J.E. Brown
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			9.5
Purchased Customer Accounts | Cover Hound
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			3.7
Purchased Customer Accounts | MAJ
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			8.0
Purchased Customer Accounts | South & Western
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			18.5
Purchased Customer Accounts | Berry
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			9.7
Purchased Customer Accounts | Other Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		11.7	8.6
Purchased customer accounts and other
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	669.2
Purchased customer accounts and other | Orchid
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	107.8
Purchased customer accounts and other | GRP
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	496.8
Purchased customer accounts and other | First
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	5.2
Purchased customer accounts and other | BdB
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	14.0
Purchased customer accounts and other | Smithwick
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	6.7
Purchased customer accounts and other | VistaNational
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	6.1
Purchased customer accounts and other | Taylor
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	8.7
Purchased customer accounts and other | Adjustments from Prior Year Acquisitions | Other Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	23.9
Non-compete Agreements
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	2.0	2.5	0.9
Non-compete Agreements | First
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	0.2
Non-compete Agreements | BdB
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	0.3
Non-compete Agreements | Smithwick
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	0.1
Non-compete Agreements | VistaNational
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	0.1
Non-compete Agreements | Taylor
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	0.3
Non-compete Agreements | O'Leary
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		0.8
Non-compete Agreements | AGIS
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		0.1
Non-compete Agreements | Winston
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		0.9
Non-compete Agreements | Remedy
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		0.5
Non-compete Agreements | Special Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			0.1
Non-compete Agreements | RLA
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			0.5
Non-compete Agreements | Buiten
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			0.1
Non-compete Agreements | VAS
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			0.1
Non-compete Agreements | Other Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		$ 0.2	$ 0.1
Non-compete Agreements | Adjustments from Prior Year Acquisitions | Other Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	$ 1.0