Accounts Receivable (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable Additional Disclosures [Abstract]
Accounts Receivable, Net, Current	$ 6,559	$ 12,731
Unbilled accounts receivable	380,819	448,088
Other Receivables	5,368	4,960
Accounts receivable, net	392,746	465,779
Allowance for Doubtful Accounts Receivable, Current	1,000	900
Unbilled accounts receivable [Abstract]
Accrued worksite employee payroll cost	334,836	401,859
Unbilled revenues	81,311	105,841
Client Prepayments Included in Cash Balance	(35,328)	(59,612)
Unbilled accounts receivable	$ 380,819	$ 448,088