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Commitments, Contingencies and Off-Balance Sheet Arrangements - Additional Information (Detail)	3 Months Ended		231 Months Ended
	Mar. 31, 2021 USD ($) ft² Entity Employee LetterOfCredit Company	Mar. 31, 2020 Entity	Mar. 31, 2021 USD ($) 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 | Employee	2,000
Number of companies acquired | Entity	25	75	589
Aggregate amount of maximum earnout obligations related to acquisitions	$ 1,146,100,000
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet	623,000,000.0
Aggregate amount of earnout obligation expected settlement in cash or stock at option	518,200,000		$ 518,200,000
Aggregate amount of earnout obligation expected settlement in cash	104,800,000		$ 104,800,000
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
Collateral [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit	1		1
Self-Insurance Deductibles [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit	2		2
Rent-A-Captive Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Number of letters of credit issued | LetterOfCredit	6		6
Errors And Omissions [Member] | Up to $4.5 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained	$ 4,500,000
Errors And Omissions [Member] | Up to $10 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained	10,000,000.0
Insurance claim arise during ordinary course of business	10,000,000.0		$ 10,000,000.0
Errors And Omissions [Member] | Up to $20.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained	20,000,000.0
Errors And Omissions [Member] | Up to $27.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claims, amount retained	27,000,000.0
Letter of Credit [Member] | Security Deposit [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt	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
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	17,600,000		17,600,000
Letter of Credit [Member] | Rent-A-Captive Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Debt	$ 4,300,000		4,300,000
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	$ 3,000,000.0		3,000,000.0
Minimum [Member] | Errors And Omissions [Member] | Up to $4.5 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business	10,000,000.0		10,000,000.0
Minimum [Member] | Errors And Omissions [Member] | Up to $20.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business	100,000,000.0		100,000,000.0
Minimum [Member] | Errors And Omissions [Member] | Up to $27.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business	240,000,000.0		240,000,000.0
Maximum [Member] | Errors And Omissions [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Actuarial range value	6,100,000		6,100,000
Maximum [Member] | Errors And Omissions [Member] | Up to $4.5 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business	100,000,000.0		100,000,000.0
Maximum [Member] | Errors And Omissions [Member] | Up to $20.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business	240,000,000.0		240,000,000.0
Maximum [Member] | Errors And Omissions [Member] | Up to $27.0 Million [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Insurance claim arise during ordinary course of business	$ 350,000,000.0		$ 350,000,000.0
Unconsolidated Investments in Enterprises [Member] | Minimum [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Ownership interest	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%

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