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Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail) | 3 Months Ended | 9 Months Ended | 12 Months Ended | 213 Months Ended
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Sep. 30, 2019 USD ($) ft² LetterOfCredit Entity | Sep. 30, 2018 Entity | Sep. 30, 2019 USD ($) ft² LetterOfCredit Employees Entity Companies | Sep. 30, 2018 Entity | Dec. 31, 2018 USD ($) | Sep. 30, 2019 USD ($) ft² LetterOfCredit Entity
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 | Employees |  |  | 2,000 |  |  |
Number of companies acquired | Entity | 29 | 38 | 86 | 95 |  | 545
Aggregate amount of maximum earnout obligations related to acquisitions |  |  | $ 764,700,000 |  |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  | 419,400,000 |  |  |
Income tax credits and adjustments |  |  | 108,000,000.0 |  |  |
Errors And Omissions [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Insurance claims, amount retained |  |  | 5,000,000.0 |  |  |
Amount of losses in excess of retained amounts | $ 350,000,000.0 |  | 350,000,000.0 |  |  | $ 350,000,000.0
Letter of Credit [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Debt | $ 900,000 |  | $ 900,000 |  |  | $ 900,000
Number of letters of credit issued | LetterOfCredit | 1 |  | 1 |  |  | 1
Letter of Credit [Member] | Security Deposit [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Debt | $ 500,000 |  | $ 500,000 |  |  | $ 500,000
Number of letters of credit issued | LetterOfCredit | 1 |  | 1 |  |  | 1
Letter of Credit [Member] | Self-Insurance Deductibles [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Debt | $ 10,100,000 |  | $ 10,100,000 |  |  | $ 10,100,000
Liabilities recorded on self-insurance | $ 16,100,000 |  | $ 16,100,000 |  |  | $ 16,100,000
Number of letters of credit issued | LetterOfCredit | 2 |  | 2 |  |  | 2
Letter of Credit [Member] | Rent-A-Captive Facility [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Debt | $ 6,300,000 |  | $ 6,300,000 |  |  | $ 6,300,000
Number of letters of credit issued | LetterOfCredit | 7 |  | 7 |  |  | 7
Minimum [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Ownership interest | 1.00% |  | 1.00% |  |  | 1.00%
Number of micro-captive insurance companies organized or managed | Companies |  |  | 100 |  |  |
Minimum [Member] | Errors And Omissions [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Actuarial range value | $ 800,000 |  | $ 800,000 |  |  | $ 800,000
Maximum [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Ownership interest | 50.00% |  | 50.00% |  |  | 50.00%
Micro-captive operations, net earnings |  |  |  |  | $ 3,200,000 |
Micro-captive operations, EBITDAC |  |  |  |  | $ 5,000,000.0 |
Maximum [Member] | Errors And Omissions [Member] |  |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |  |
Actuarial range value | $ 8,600,000 |  | $ 8,600,000 |  |  | $ 8,600,000

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