Business Combinations - Acquisition Method for Recording Business Combinations (Detail) shares in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) shares
First Ireland Risk Management Ltd [Member]
Business Acquisition [Line Items]
Cash Paid	$ 86,400,000
Escrow Deposited	5,300,000
Recorded Earnout Payable	6,100,000
Total Recorded Purchase Price	97,800,000
Maximum Potential Earnout Payable	6,600,000
BCHR Holdings, L.P. dba Buck [Member]
Business Acquisition [Line Items]
Cash Paid	600,800,000
Accrued Liability	500,000
Escrow Deposited	19,500,000
Total Recorded Purchase Price	$ 620,800,000
Boley-Featherston-Huffman & Deal Co. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	243
Common Share Value	$ 45,200,000
Cash Paid	8,800,000
Escrow Deposited	6,000,000
Recorded Earnout Payable	9,300,000
Total Recorded Purchase Price	69,300,000
Maximum Potential Earnout Payable	15,000,000
Tay River Holdings Limited [Member]
Business Acquisition [Line Items]
Cash Paid	40,300,000
Accrued Liability	4,300,000
Escrow Deposited	2,400,000
Recorded Earnout Payable	32,000,000.0
Total Recorded Purchase Price	79,000,000.0
Maximum Potential Earnout Payable	$ 88,700,000
Insurance By Ken Brown [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	273
Common Share Value	$ 53,900,000
Cash Paid	1,800,000
Escrow Deposited	2,900,000
Recorded Earnout Payable	11,300,000
Total Recorded Purchase Price	69,900,000
Maximum Potential Earnout Payable	$ 17,500,000
RHP General Agency [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	335
Common Share Value	$ 65,700,000
Cash Paid	1,300,000
Escrow Deposited	5,000,000
Recorded Earnout Payable	4,400,000
Total Recorded Purchase Price	76,400,000
Maximum Potential Earnout Payable	$ 11,000,000
Clements & Co [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	193
Common Share Value	$ 43,300,000
Cash Paid	7,100,000
Escrow Deposited	3,000,000
Recorded Earnout Payable	30,900,000
Total Recorded Purchase Price	84,300,000
Maximum Potential Earnout Payable	70,000,000
Eastern Insurance Group, LLC [Member]
Business Acquisition [Line Items]
Cash Paid	511,400,000
Accrued Liability	3,700,000
Total Recorded Purchase Price	515,100,000
Cadence Insurance, Inc. [Member]
Business Acquisition [Line Items]
Cash Paid	880,100,000
Accrued Liability	5,900,000
Total Recorded Purchase Price	886,000,000
My Plan Manager [Member]
Business Acquisition [Line Items]
Cash Paid	298,600,000
Escrow Deposited	3,000,000
Total Recorded Purchase Price	$ 301,600,000
Forty-one Other Acquisitions [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	450
Common Share Value	$ 98,000,000
Cash Paid	671,600,000
Accrued Liability	11,700,000
Escrow Deposited	47,600,000
Recorded Earnout Payable	113,500,000
Total Recorded Purchase Price	942,400,000
Maximum Potential Earnout Payable	$ 191,400,000
2023 Acquisitions [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	1,494
Common Share Value	$ 306,100,000
Cash Paid	3,108,200,000
Accrued Liability	26,100,000
Escrow Deposited	94,700,000
Recorded Earnout Payable	207,500,000
Total Recorded Purchase Price	3,742,600,000
Maximum Potential Earnout Payable	$ 400,200,000