Business Combinations - Acquisition Method for Recording Business Combinations (Detail) shares in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) shares
Business Acquisition [Line Items]
Common Shares Issued | shares	826
Common Share Value	$ 54,200,000
Cash Paid	792,200,000
Accrued Liability	3,100,000
Escrow Deposited	71,400,000
Recorded Earnout Payable	94,000,000
Total Recorded Purchase Price	1,014,900,000
Maximum Potential Earnout Payable	$ 201,500,000
Market Financial Group, Ltd and Austin Consulting Group Inc. (MFG) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	53
Common Share Value	$ 3,700,000
Cash Paid	33,900,000
Escrow Deposited	4,200,000
Recorded Earnout Payable	2,700,000
Total Recorded Purchase Price	44,500,000
Maximum Potential Earnout Payable	7,000,000
McGregor & Associates (M&A) [Member]
Business Acquisition [Line Items]
Cash Paid	13,500,000
Escrow Deposited	2,500,000
Recorded Earnout Payable	5,100,000
Total Recorded Purchase Price	21,100,000
Maximum Potential Earnout Payable	12,000,000
Pronto Insurance (PI) [Member]
Business Acquisition [Line Items]
Cash Paid	294,800,000
Escrow Deposited	18,700,000
Total Recorded Purchase Price	$ 313,500,000
Reassurance Holdings, Inc (RHI) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	343
Common Share Value	$ 24,400,000
Cash Paid	84,400,000
Escrow Deposited	13,300,000
Recorded Earnout Payable	3,300,000
Total Recorded Purchase Price	125,400,000
Maximum Potential Earnout Payable	21,500,000
Buckman Mitchell, Inc. (BMI) [Member]
Business Acquisition [Line Items]
Cash Paid	43,900,000
Escrow Deposited	1,900,000
Recorded Earnout Payable	3,600,000
Total Recorded Purchase Price	49,400,000
Maximum Potential Earnout Payable	8,500,000
Accompass Inc. (AI) [Member]
Business Acquisition [Line Items]
Cash Paid	29,800,000
Escrow Deposited	2,600,000
Recorded Earnout Payable	4,400,000
Total Recorded Purchase Price	36,800,000
Maximum Potential Earnout Payable	$ 11,300,000
42 Other Acquisitions [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	430
Common Share Value	$ 26,100,000
Cash Paid	291,900,000
Accrued Liability	3,100,000
Escrow Deposited	28,200,000
Recorded Earnout Payable	74,900,000
Total Recorded Purchase Price	424,200,000
Maximum Potential Earnout Payable	$ 141,200,000