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Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail)
 12 Months Ended 192 Months Ended
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