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Business Combinations - Acquisition Method for Recording Business Combinations (Detail) shares in Thousands | 12 Months Ended
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Dec. 31, 2020 USD ($) shares
Business Acquisition [Line Items] |
Common Shares Issued | shares | 1,843
Common Share Value | $ 189,700,000
Cash Paid | 398,300,000
Accrued Liability | 50,000,000.0
Escrow Deposited | 30,700,000
Recorded Earnout Payable | 207,400,000
Total Recorded Purchase Price | 876,100,000
Maximum Potential Earnout Payable | $ 387,900,000
Capsicum Reinsurance Brokers LLP (CRB) [Member] |
Business Acquisition [Line Items] |
Common Shares Issued | shares | 584
Common Share Value | $ 62,900,000
Cash Paid | 64,500,000
Recorded Earnout Payable | 129,900,000
Total Recorded Purchase Price | 257,300,000
Maximum Potential Earnout Payable | 209,100,000
Hanover Excess & Surplus, Inc. and Hanover Premium Finance, Inc. (HES) [Member] |
Business Acquisition [Line Items] |
Cash Paid | 30,100,000
Escrow Deposited | 3,000,000.0
Total Recorded Purchase Price | 33,100,000
Maximum Potential Earnout Payable | $ 9,300,000
CRES Insurance Services, LLC (CRES) [Member] |
Business Acquisition [Line Items] |
Common Shares Issued | shares | 288
Common Share Value | $ 28,500,000
Cash Paid | 1,500,000
Escrow Deposited | 1,000,000.0
Recorded Earnout Payable | 5,500,000
Total Recorded Purchase Price | 36,500,000
Maximum Potential Earnout Payable | $ 7,300,000
Optimum Talent Inc. (OTI) [Member] |
Business Acquisition [Line Items] |
Common Shares Issued | shares | 102
Common Share Value | $ 11,100,000
Cash Paid | 14,100,000
Escrow Deposited | 3,400,000
Recorded Earnout Payable | 14,000,000.0
Total Recorded Purchase Price | 42,600,000
Maximum Potential Earnout Payable | 21,100,000
Ashton Tiffany, LLC (AT) [Member] |
Business Acquisition [Line Items] |
Cash Paid | 48,300,000
Accrued Liability | 48,200,000
Recorded Earnout Payable | 9,000,000.0
Total Recorded Purchase Price | 105,500,000
Maximum Potential Earnout Payable | $ 20,000,000.0
Cool Insuring Agency, Inc. (CIA) [Member] |
Business Acquisition [Line Items] |
Common Shares Issued | shares | 406
Common Share Value | $ 48,400,000
Cash Paid | 65,000,000.0
Escrow Deposited | 7,200,000
Recorded Earnout Payable | 8,500,000
Total Recorded Purchase Price | 129,100,000
Maximum Potential Earnout Payable | $ 30,000,000.0
Harden & Associates, Inc. (HAI) [Member] |
Business Acquisition [Line Items] |
Common Shares Issued | shares | 49
Cash Paid | $ 95,600,000
Escrow Deposited | 6,200,000
Recorded Earnout Payable | 10,000,000.0
Total Recorded Purchase Price | 111,800,000
Maximum Potential Earnout Payable | $ 22,500,000
Twenty Other Acquisitions [Member] |
Business Acquisition [Line Items] |
Common Shares Issued | shares | 414
Common Share Value | $ 38,800,000
Cash Paid | 79,200,000
Accrued Liability | 1,800,000
Escrow Deposited | 9,900,000
Recorded Earnout Payable | 30,500,000
Total Recorded Purchase Price | 160,200,000
Maximum Potential Earnout Payable | $ 68,600,000

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