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Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail) | 3 Months Ended | 9 Months Ended | 201 Months Ended
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Sep. 30, 2018 USD ($) ft² LetterOfCredit Entity | Sep. 30, 2017 Entity | Sep. 30, 2018 USD ($) ft² LetterOfCredit Entity Employees | Sep. 30, 2017 Entity | Sep. 30, 2018 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 |  |
Acquired property through lease area, sqft | ft² |  |  | 306,000 |  |
Acquired property through lease percentage of building |  |  | 60.00% |  |
Operating lease commitment, expiration date |  |  | Feb. 28, 2018 |  |
Number of companies acquired | Entity | 38 | 31 | 95 | 89 | 488
Aggregate amount of maximum earnout obligations related to acquisitions | $ 495,700,000 |  | $ 495,700,000 |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet | 219,600,000 |  | $ 219,600,000 |  |
Tax Increment Financing [Member] |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |
Property tax credit expect to receive period |  |  | 15 years |  |
Economic development for growing economy tax credit | 11,100,000 |  | $ 11,100,000 |  | $ 11,100,000
Letter of Credit [Member] | Collateral Related to Claim Funds [Member] |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |
Debt | $ 1,300,000 |  | $ 1,300,000 |  | $ 1,300,000
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 | $ 9,700,000 |  | $ 9,700,000 |  | $ 9,700,000
Liabilities recorded on self-insurance | $ 14,800,000 |  | $ 14,800,000 |  | $ 14,800,000
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%
Minimum [Member] | Tax Increment Financing [Member] |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |
Property tax credit expect to receive | $ 60,000,000 |  | $ 60,000,000 |  | $ 60,000,000
Maximum [Member] |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |
Ownership interest | 50.00% |  | 50.00% |  | 50.00%
Maximum [Member] | Tax Increment Financing [Member] |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |
Property tax credit expect to receive | $ 80,000,000 |  | $ 80,000,000 |  | $ 80,000,000
Brokerage [Member] | United Kingdom [Member] |  |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |  |
Sublease arrangement, term | 10 years |  |  |  |
Sublease arrangement | $ 22,000,000 |  |  |  |

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