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Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail)	3 Months Ended		9 Months Ended		225 Months Ended
	Sep. 30, 2020 USD ($) ft² Entity LetterOfCredit	Sep. 30, 2019 Entity	Sep. 30, 2020 USD ($) ft² Entity Employee LetterOfCredit Company	Sep. 30, 2019 Entity	Sep. 30, 2020 USD ($) ft² Entity LetterOfCredit
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 | Employee			2,000
Number of companies acquired | Entity	55	29	126	86	573
Aggregate amount of maximum earnout obligations related to acquisitions			$ 1,026,100,000
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet			502,500,000
Income tax credits and adjustments			$ 108,000,000.0
Security Deposit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit	1		1		1
Collateral [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit	1		1		1
Self-Insurance Deductibles [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit	2		2		2
Rent-A-Captive Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit	7		7		7
Errors And Omissions [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained			$ 10,000,000.0
Amount of losses in excess of retained amounts	$ 350,000,000.0		350,000,000.0		$ 350,000,000.0
Letter of Credit [Member] | Security Deposit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt	500,000		500,000		500,000
Letter of Credit [Member] | Collateral [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt	1,000,000.0		1,000,000.0		1,000,000.0
Letter of Credit [Member] | Self-Insurance Deductibles [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt	9,400,000		9,400,000		9,400,000
Liabilities recorded on self-insurance	17,300,000		17,300,000		17,300,000
Letter of Credit [Member] | Rent-A-Captive Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt	7,500,000		$ 7,500,000		7,500,000
Minimum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of micro-captive insurance companies organized or managed | Company			100
Minimum [Member] | Errors And Omissions [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Actuarial range value	3,700,000		$ 3,700,000		3,700,000
Maximum [Member] | Errors And Omissions [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Actuarial range value	$ 5,100,000		$ 5,100,000		$ 5,100,000
Unconsolidated Investments in Enterprises [Member] | Minimum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Ownership interest	1.00%		1.00%		1.00%
Unconsolidated Investments in Enterprises [Member] | Maximum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Ownership interest	50.00%		50.00%		50.00%

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