Business Combinations - Acquisition Method for Recording Business Combinations (Detail) shares in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) shares
Business Acquisition [Line Items]
Common Shares Issued | shares	1,787
Common Share Value	$ 155,900,000
Cash Paid	1,274,500,000
Accrued Liability	16,400,000
Escrow Deposited	72,800,000
Recorded Earnout Payable	272,200,000
Total Recorded Purchase Price	1,791,800,000
Maximum Potential Earnout Payable	$ 477,100,000
Inverson Holding Company - Company, LLC (IHC) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	452
Common Share Value	$ 35,900,000
Cash Paid	31,200,000
Escrow Deposited	4,500,000
Recorded Earnout Payable	20,800,000
Total Recorded Purchase Price	92,400,000
Maximum Potential Earnout Payable	35,000,000.0
James Brown Inc. (JBI) [Member]
Business Acquisition [Line Items]
Cash Paid	65,900,000
Escrow Deposited	8,700,000
Total Recorded Purchase Price	74,600,000
Limited SPG [Member]
Business Acquisition [Line Items]
Cash Paid	326,800,000
Escrow Deposited	4,800,000
Total Recorded Purchase Price	331,600,000
RPA Insurance Services LLC [Member]
Business Acquisition [Line Items]
Cash Paid	44,000,000.0
Escrow Deposited	3,900,000
Recorded Earnout Payable	16,900,000
Total Recorded Purchase Price	64,800,000
Maximum Potential Earnout Payable	22,000,000.0
JLT Aerospace [Member]
Business Acquisition [Line Items]
Cash Paid	162,800,000
Recorded Earnout Payable	67,900,000
Total Recorded Purchase Price	230,700,000
Maximum Potential Earnout Payable	75,100,000
RGA Group [Member]
Business Acquisition [Line Items]
Cash Paid	42,800,000
Accrued Liability	6,000,000.0
Escrow Deposited	4,700,000
Recorded Earnout Payable	8,000,000.0
Total Recorded Purchase Price	61,500,000
Maximum Potential Earnout Payable	$ 9,300,000
LSG Insurance Partners, Inc. (LSG) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	395
Common Share Value	$ 36,200,000
Cash Paid	127,200,000
Escrow Deposited	6,000,000.0
Recorded Earnout Payable	50,100,000
Total Recorded Purchase Price	219,500,000
Maximum Potential Earnout Payable	$ 71,500,000
The EHE Group, LLC dba BonusDrive (EHE) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	458
Common Share Value	$ 42,000,000.0
Cash Paid	41,000,000.0
Accrued Liability	200,000
Escrow Deposited	1,000,000.0
Recorded Earnout Payable	27,000,000.0
Total Recorded Purchase Price	111,200,000
Maximum Potential Earnout Payable	77,000,000.0
Horseshoe Insurance Services Holdings, Ltd. (HIS) [Member]
Business Acquisition [Line Items]
Cash Paid	43,300,000
Accrued Liability	8,500,000
Escrow Deposited	5,000,000.0
Recorded Earnout Payable	7,100,000
Total Recorded Purchase Price	63,900,000
Maximum Potential Earnout Payable	$ 10,000,000.0
40 other acquisitions [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	482
Common Share Value	$ 41,800,000
Cash Paid	389,500,000
Accrued Liability	1,700,000
Escrow Deposited	34,200,000
Recorded Earnout Payable	74,400,000
Total Recorded Purchase Price	541,600,000
Maximum Potential Earnout Payable	$ 177,200,000