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Business Combinations - Acquisition Method for Recording Business Combinations (Detail) shares in Thousands, $ in Millions | 3 Months Ended | 6 Months Ended | 282 Months Ended
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Jun. 30, 2025 USD ($) acquisition | Jun. 30, 2024 acquisition | Jun. 30, 2025 USD ($) acquisition shares | Jun. 30, 2024 acquisition | Jun. 30, 2025 USD ($) company
Business Acquisition [Line Items] |  |  |  |  |
Number of companies acquired | 36 | 30 | 56 | 58 | 770
2025 Acquisitions |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Common Shares Issued (in shares) | shares |  |  | 49 |  |
Common Shares Value |  |  | $ 16.0 |  |
Cash Paid |  |  | 1,651.7 |  |
Accrued Liability | $ 36.3 |  | 36.3 |  | $ 36.3
Escrow Deposited | 83.9 |  | 83.9 |  | 83.9
Recorded Earnout Payable |  |  | 78.9 |  |
Total Recorded Purchase Price | 1,866.8 |  | 1,866.8 |  | 1,866.8
Maximum Potential Earnout Payable | 208.0 |  | $ 208.0 |  | 208.0
W K Webster & Co Ltd February 1, 2025 (WKW) |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Common Shares Issued (in shares) | shares |  |  | 0 |  |
Common Shares Value |  |  | $ 0.0 |  |
Cash Paid |  |  | 138.4 |  |
Accrued Liability | 1.9 |  | 1.9 |  | 1.9
Escrow Deposited | 0.0 |  | 0.0 |  | 0.0
Recorded Earnout Payable |  |  | 13.9 |  |
Total Recorded Purchase Price | 154.2 |  | 154.2 |  | 154.2
Maximum Potential Earnout Payable | 28.5 |  | $ 28.5 |  | 28.5
Case Group February 26, 2025 (CSG) |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Common Shares Issued (in shares) | shares |  |  | 0 |  |
Common Shares Value |  |  | $ 0.0 |  |
Cash Paid |  |  | 57.8 |  |
Accrued Liability | 3.5 |  | 3.5 |  | 3.5
Escrow Deposited | 6.5 |  | 6.5 |  | 6.5
Recorded Earnout Payable |  |  | 15.0 |  |
Total Recorded Purchase Price | 82.8 |  | 82.8 |  | 82.8
Maximum Potential Earnout Payable | 84.1 |  | $ 84.1 |  | 84.1
Woodruff Sawyer & Co April 10, 2025 (WSC) |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Common Shares Issued (in shares) | shares |  |  | 0 |  |
Common Shares Value |  |  | $ 0.0 |  |
Cash Paid |  |  | 1,195.5 |  |
Accrued Liability | 12.2 |  | 12.2 |  | 12.2
Escrow Deposited | 66.6 |  | 66.6 |  | 66.6
Recorded Earnout Payable |  |  | 0.0 |  |
Total Recorded Purchase Price | 1,274.3 |  | 1,274.3 |  | 1,274.3
Maximum Potential Earnout Payable | 0.0 |  | $ 0.0 |  | 0.0
Seventeen Other Acquisitions |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Common Shares Issued (in shares) | shares |  |  | 49 |  |
Common Shares Value |  |  | $ 16.0 |  |
Cash Paid |  |  | 260.0 |  |
Accrued Liability | 18.7 |  | 18.7 |  | 18.7
Escrow Deposited | 10.8 |  | 10.8 |  | 10.8
Recorded Earnout Payable |  |  | 50.0 |  |
Total Recorded Purchase Price | 355.5 |  | 355.5 |  | 355.5
Maximum Potential Earnout Payable | $ 95.4 |  | $ 95.4 |  | $ 95.4
Number of companies acquired | acquisition |  |  | 17 |  |

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