Accounts Receivable (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Accounts Receivable [Abstract]
Trade receivables, allowance for doubtful accounts	$ 1,000,000
Accounts receivable, due date prior to applicable payroll date	1 day
Unbilled accounts receivable [Abstract]
Accrued worksite employee payroll cost	150,070,000	130,317,000
Unbilled revenues	44,483,000	38,564,000
Customer prepayments	(13,513,000)	(10,373,000)
Unbilled accounts receivable	$ 181,040,000	$ 158,508,000