Accounts Receivable (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Accounts Receivable [Abstract]
Trade receivables, allowance for doubtful accounts	$ 1,000	$ 988
Accounts receivable, due date prior to applicable payroll date (in days)	1D
Unbilled accounts receivable [Abstract]
Unbilled accounts receivable	158,508	134,187
Accrued Worksite Employee Payroll Cost [Member]
Unbilled accounts receivable [Abstract]
Accrued worksite employee payroll cost	130,317	109,697
Unbilled revenues	38,564	32,613
Customer prepayments	(10,373)	(8,123)
Unbilled accounts receivable	$ 158,508	$ 134,187