Accounts Receivable (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Accounts Receivable Additional Disclosures [Abstract]
Trade receivables, allowance for doubtful accounts	$ 1,200,000	$ 900,000
Accounts receivable, due date prior to applicable payroll date	1 day
Unbilled accounts receivable [Abstract]
Accrued worksite employee payroll cost	192,396,000	173,801,000
Unbilled revenues	55,645,000	50,286,000
Customer prepayments	(87,887,000)	(24,459,000)
Unbilled accounts receivable	$ 160,154,000	$ 199,628,000