Worksite Employee-Related Assets and Liabilities - Schedule of Components of the Company's WSE-Related Assets and WSE-Related Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Worksite employee-related assets:
Restricted cash	$ 14,550	$ 14,543
Total worksite employee-related assets	838,239	1,635,136
Worksite employee-related liabilities:
Health benefits payable	26,250	29,179
Workers compensation payable	42,935	36,778
Total worksite employee-related liabilities	832,531	1,630,555
WSE
Worksite employee-related assets:
Restricted cash	82,611	64,890
Restricted investments	2,318	4,555
Payroll funds collected	435,033	1,336,994
Unbilled revenue, net of advance collections of $80,719 and $113,190 at June 30, 2015 and December 31, 2014, respectively	289,240	203,599
Accounts receivable, net of allowance for doubtful accounts of $548 and $388 at June 30, 2015 and December 31, 2014, respectively	9,848	5,193
Prepaid health plan expenses	6,029	4,932
Prepaid workers compensation expenses	3,304	1,256
Other payroll assets	3,584	5,742
Total worksite employee-related assets	838,239	1,635,136
Worksite employee-related liabilities:
Unbilled wages accrual	347,016	292,906
Payroll taxes payable	250,447	1,119,427
Health benefits payable	122,914	104,220
Customer prepayments	41,461	53,770
Workers compensation payable	42,935	36,778
Other payroll deductions	27,758	23,454
Total worksite employee-related liabilities	832,531	1,630,555
WSE | Workers Compensation
Worksite employee-related assets:
Refundable premiums	$ 6,272	$ 7,975