Business Combinations - Purchase Price Allocation for Current Year Acquisitions and Adjustments Made for Prior Year Acquisitions (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Mar. 31, 2021	Sep. 30, 2022
Business Combination Separately Recognized Transactions [Line Items]
Cash paid			$ 2,504.7
Common stock issued	$ 14.7	$ 4.9	14.7
Other payable			4.6
Recorded earn-out payable			42.1
Net assets acquired	2,566.1		2,566.1
Orchid Intermediate Holdings L P
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	487.0		$ 487.0
Orchid Intermediate Holdings L P | National Programs
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Mar. 31, 2022
Cash paid			$ 476.2
Common stock issued			0.0
Other payable			0.0
Recorded earn-out payable			10.8
Net assets acquired	487.0		487.0
Maximum potential earn- out payable	20.0		20.0
GRP (Jersey) Holdco Limited (GRP)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	1,854.5		$ 1,854.5
GRP (Jersey) Holdco Limited (GRP) | Retail National Programs Wholesale
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Jul. 01, 2022
Cash paid			$ 1,839.8
Common stock issued			14.7
Other payable			0.0
Recorded earn-out payable			0.0
Net assets acquired	1,854.5		1,854.5
Maximum potential earn- out payable	0.0		0.0
First Insurance Solutions Ltd. (First)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	22.7		$ 22.7
First Insurance Solutions Ltd. (First) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Jul. 08, 2022
Cash paid			$ 13.0
Common stock issued			0.0
Other payable			1.4
Recorded earn-out payable			8.3
Net assets acquired	22.7		22.7
Maximum potential earn- out payable	8.4		8.4
BdB Holdings Limited (BdB)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	86.7		$ 86.7
BdB Holdings Limited (BdB) | Wholesale
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Aug. 01, 2022
Cash paid			$ 75.3
Common stock issued			0.0
Other payable			0.0
Recorded earn-out payable			11.4
Net assets acquired	86.7		86.7
Maximum potential earn- out payable	36.3		36.3
Smithwick & Mariners Insurance, Inc. (Smithwick)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	25.8		$ 25.8
Smithwick & Mariners Insurance, Inc. (Smithwick) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Sep. 01, 2022
Cash paid			$ 23.2
Common stock issued			0.0
Other payable			1.0
Recorded earn-out payable			1.6
Net assets acquired	25.8		25.8
Maximum potential earn- out payable	6.5		6.5
VistaNational Insurance Group, Inc. (VistaNational)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	28.6		$ 28.6
VistaNational Insurance Group, Inc. (VistaNational) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Sep. 01, 2022
Cash paid			$ 26.7
Common stock issued			0.0
Other payable			0.8
Recorded earn-out payable			1.1
Net assets acquired	28.6		28.6
Maximum potential earn- out payable	3.0		3.0
Other Acquisitions | Various
Business Combination Separately Recognized Transactions [Line Items]
Cash paid			50.5
Common stock issued			0.0
Other payable			1.4
Recorded earn-out payable			8.9
Net assets acquired	60.8		60.8
Maximum potential earn- out payable	17.9		17.9
2022 Acquisition
Business Combination Separately Recognized Transactions [Line Items]
Maximum potential earn- out payable	$ 92.1		$ 92.1