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Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail) |  | 3 Months Ended | 9 Months Ended | 237 Months Ended
--- | --- | --- | --- | ---
Jul. 29, 2021 | Sep. 30, 2021 USD ($) ft² Entity LetterOfCredit | Sep. 30, 2020 Entity | Sep. 30, 2021 USD ($) ft² Entity Employee LetterOfCredit | Sep. 30, 2020 USD ($) Entity | Sep. 30, 2021 USD ($) ft² Entity LetterOfCredit
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Number of square feet | ft² |  | 360,000 |  | 360,000 |  | 360,000
Number of employees will accommodate at new facility | Employee |  |  |  | 2,000 |  |
Number of companies acquired | Entity |  | 44 | 55 | 80 | 126 | 602
Aggregate amount of maximum earnout obligations related to acquisitions |  |  |  | $ 1,026,100,000 | $ 1,026,100,000 |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  |  | 578,700,000 | $ 578,700,000 |
Aggregate amount of earnout obligation expected settlement in cash or stock at option |  | $ 560,500,000 |  | 560,500,000 |  | $ 560,500,000
Aggregate amount of earnout obligation expected settlement in cash |  | $ 18,200,000 |  | 18,200,000 |  | $ 18,200,000
Income tax credits and adjustments |  |  |  | $ 108,000,000.0 |  |
Security Deposit [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Number of letters of credit issued | LetterOfCredit |  | 1 |  | 1 |  | 1
Collateral [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Number of letters of credit issued | LetterOfCredit |  | 1 |  | 1 |  | 1
Self-Insurance Deductibles [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Number of letters of credit issued | LetterOfCredit |  | 2 |  | 2 |  | 2
Rent-A-Captive Facility [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Number of letters of credit issued | LetterOfCredit |  | 6 |  | 6 |  | 6
Errors And Omissions [Member] | Up to $4.5 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claims, amount retained |  |  |  | $ 2,000,000.0 |  |
Errors And Omissions [Member] | Up to $10 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claims, amount retained |  |  |  | 15,000,000.0 |  |
Insurance claim arise during ordinary course of business |  | $ 15,000,000.0 |  | 15,000,000.0 |  | $ 15,000,000.0
Errors And Omissions [Member] | Up to $20.0 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claims, amount retained |  |  |  | 20,000,000.0 |  |
Errors And Omissions [Member] | Up to $27.0 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claims, amount retained |  |  |  | 10,000,000.0 |  |
Letter of Credit [Member] | Security Deposit [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Debt |  | 500,000 |  | 500,000 |  | 500,000
Letter of Credit [Member] | Collateral [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Debt |  | 900,000 |  | 900,000 |  | 900,000
Letter of Credit [Member] | Self-Insurance Deductibles [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Debt |  | 9,400,000 |  | 9,400,000 |  | 9,400,000
Liabilities recorded on self-insurance |  | 18,700,000 |  | 18,700,000 |  | 18,700,000
Letter of Credit [Member] | Rent-A-Captive Facility [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Debt |  | 4,300,000 |  | 4,300,000 |  | 4,300,000
Minimum [Member] | Errors And Omissions [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Actuarial range value |  | 2,800,000 |  | 2,800,000 |  | 2,800,000
Minimum [Member] | Errors And Omissions [Member] | Up to $4.5 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claim arise during ordinary course of business |  | 15,000,000.0 |  | 15,000,000.0 |  | 15,000,000.0
Minimum [Member] | Errors And Omissions [Member] | Up to $20.0 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claim arise during ordinary course of business |  | 100,000,000.0 |  | 100,000,000.0 |  | 100,000,000.0
Minimum [Member] | Errors And Omissions [Member] | Up to $27.0 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claim arise during ordinary course of business |  | 225,000,000.0 |  | 225,000,000.0 |  | 225,000,000.0
Maximum [Member] | Errors And Omissions [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Actuarial range value |  | 7,400,000 |  | 7,400,000 |  | 7,400,000
Maximum [Member] | Errors And Omissions [Member] | Up to $4.5 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claim arise during ordinary course of business |  | 100,000,000.0 |  | 100,000,000.0 |  | 100,000,000.0
Maximum [Member] | Errors And Omissions [Member] | Up to $20.0 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claim arise during ordinary course of business |  | 225,000,000.0 |  | 225,000,000.0 |  | 225,000,000.0
Maximum [Member] | Errors And Omissions [Member] | Up to $27.0 Million [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claim arise during ordinary course of business |  | $ 365,000,000.0 |  | $ 365,000,000.0 |  | $ 365,000,000.0
Unconsolidated Investments in Enterprises [Member] | Minimum [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Ownership interest |  | 1.00% |  | 1.00% |  | 1.00%
Unconsolidated Investments in Enterprises [Member] | Maximum [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Ownership interest |  | 50.00% |  | 50.00% |  | 50.00%
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Notes redemption date | Aug. 13, 2021 |  |  |  |  |
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Private Placement [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Notes redemption date | Aug. 13, 2021 |  |  |  |  |

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