Worksite Employee Related Assets and Liabilities - Schedule of Components of the Company's WSE-Related Assets and WSE-Related Liabilities (Details) $ in Thousands	Mar. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($) employee
Worksite employee-related assets:
Restricted cash, cash equivalents and investments	$ 139,175	$ 132,115
Payroll funds collected	580,515	825,958
Unbilled revenue (net of advance collections of $8,232 and $8,602 at March 31, 2017 and December 31, 2016, respectively)	279,753	293,192
Accounts receivable (net of allowance for doubtful accounts of $316 and $292 at March 31, 2017 and December 31, 2016, respectively)	11,478	4,854
Prepaid insurance premiums	14,013	12,805
Workers' compensation collateral receivable	4,106	2,136
Other payroll assets	10,309	10,411
Total worksite employee-related assets	1,039,349	1,281,471
Advance collection	8,232	8,602
Allowance for doubtful accounts	316	292
Worksite employee-related liabilities:
Accrued wages	265,515	272,966
Client deposits	67,350	56,182
Payroll tax liabilities	449,005	692,460
Health benefits loss reserves	126,960	129,430
Workers' compensation loss reserves (net of collateral paid of $5,831 and $9,234 at March 31, 2017 and December 31, 2016, respectively)	69,167	63,702
Collateral paid to insurance carriers, net of workers' compensation loss reserves, current	5,831	9,234
Insurance premiums and other payables	10,812	14,223
Other payroll deductions	45,284	47,032
Total worksite employee-related liabilities	$ 1,034,093	$ 1,275,995
Number of employees | employee	2,600	2,600