Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail)	12 Months Ended			216 Months Ended
	Dec. 31, 2019 USD ($) ft² Companies Employees LetterOfCredit Entity	Dec. 31, 2018 USD ($) Entity	Dec. 31, 2017 USD ($) Entity	Dec. 31, 2019 USD ($) ft² LetterOfCredit Entity
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
Lease abandonment charges			$ 13,200,000
Total rent expense	$ 148,100,000	$ 140,000,000.0	$ 137,700,000
Number of companies acquired | Entity	116	112	108	556
Aggregate amount of maximum earnout obligations related to acquisitions	$ 982,900,000	$ 558,100,000
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet	$ 565,000,000.0	$ 258,800,000
Ownership interest	50.00%			50.00%
Debt	$ 18,500,000			$ 18,500,000
Liabilities recorded on self-insurance	16,500,000			16,500,000
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
Tax Increment Financing [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Economic development for growing economy tax credit	18,100,000			18,100,000
Letter of Credit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt	$ 900,000			$ 900,000
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	$ 9,400,000			$ 9,400,000
Liabilities recorded on self-insurance	$ 16,500,000			$ 16,500,000
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%
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	$ 2,400,000			$ 2,400,000
Minimum [Member] | Tax Increment Financing [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Property tax credit expect to receive	$ 60,000,000.0			$ 60,000,000.0
Maximum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Ownership interest	50.00%			50.00%
Micro-captive operations, net earnings	$ 2,900,000
Micro-captive operations, EBITDAC	4,500,000
Maximum [Member] | Errors And Omissions [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Actuarial range value	4,200,000			$ 4,200,000
Maximum [Member] | Tax Increment Financing [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Property tax credit expect to receive	$ 90,000,000.0			$ 90,000,000.0