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Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail) | 12 Months Ended | 192 Months Ended
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Dec. 31, 2017 USD ($) ft² Entity LetterOfCredit Employees | Dec. 31, 2016 USD ($) Entity | Dec. 31, 2015 USD ($) Entity | Dec. 31, 2017 USD ($) ft² Entity LetterOfCredit
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |
Number of square feet | ft² | 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 |  |  |
Lease abandonment charges | $ 13,200,000 |  |  |
Total rent expense | $ 137,700,000 | $ 134,200,000 | $ 121,600,000 |
Number of companies acquired | Entity | 108 | 101 | 105 | 459
Aggregate amount of maximum earnout obligations related to acquisitions | $ 567,900,000 | $ 527,200,000 |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet | $ 264,200,000 | $ 242,300,000 |  |
Ownership interest | 50.00% |  |  | 50.00%
Debt | $ 19,700,000 |  |  | $ 19,700,000
Liabilities recorded on self-insurance | $ 13,500,000 |  |  | 13,500,000
Tax Increment Financing [Member] |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |
Property tax credit expect to receive period | 15 years |  |  |
Letter of Credit [Member] |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |
Debt | $ 700,000 |  |  | 700,000
Liabilities recorded on self-insurance | $ 700,000 |  |  | $ 700,000
Number of letters of credit issued | LetterOfCredit | 1 |  |  | 1
Letter of Credit [Member] | Security Deposit [Member] |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |
Debt | $ 500,000 |  |  | $ 500,000
Number of letters of credit issued | LetterOfCredit | 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
Liabilities recorded on self-insurance | $ 12,800,000 |  |  | $ 12,800,000
Number of letters of credit issued | LetterOfCredit | 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
Number of letters of credit issued | LetterOfCredit | 7 |  |  | 7
Minimum [Member] |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |
Ownership interest | 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
Maximum [Member] |  |  |  |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |  |  |  |
Ownership interest | 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

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