Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Description of Business [Abstract]
Number of years transition to comprehensive business performance solutions provider	25 years
Percentage of PEO revenues from the Company's Texas markets (in hundredths)	26.00%	27.00%	28.00%
Percentage of PEO revenues from the Company's California markets (in hundredths)	17.00%	16.00%	15.00%
Number of reportable segments	1
Property and Equipment, Estimated Useful Lives [Abstract]
Amortization of capitalized computer software costs	$ 2,800,000	$ 1,600,000	$ 1,600,000
Unamortized computer software costs	8,200,000	7,600,000
Health Insurance Costs [Abstract]
Number of days cash in advance of beginning of reporting quarter United establishes cash funding rates	90 days
Required accumulated cash surplus	9,000,000
Amount which Plan Costs were less than the net premiums paid and owed	18,500,000
Prepaid health insurance, current	9,500,000
Premiums owed to United	10,500,000
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence	1,000,000
Company's maximum aggregate economic burden for claims in excess of $1 million per policy year	5,000,000
Reduction in accrued workers' compensation costs for changes in estimated losses	13,100,000	11,400,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	0.60%	1.10%
Stock-Based Compensation [Abstract]
Number of stock-based employee compensation plans	3
Vesting period for officers and other management to be eligible for restricted stock grants, minimum	3 years
Vesting period for officers and other management to be eligible for restricted stock grants, maximum	5 years
Percentage of grants issued to directors that are vested (in hundredths)	100.00%
Company-Sponsored 401 (k) Plans [Abstract]
Percentage match of eligible corporate employees' contributions (in hundredths)	50.00%
Percentage of eligible compensation matched, minimum (in hundredths)	0.00%
Percentage of eligible compensation matched, maximum (in hundredths)	6.00%
Incurred but not paid workers' compensation liabilities
Beginning balance	104,791,000	96,934,000
Accrued claims	37,772,000	36,845,000
Present value discount	(868,000)	(1,513,000)
Paid claims	(30,010,000)	(27,475,000)
Current portion of accrued claims	47,149,000	44,737,000
Long-term portion of accrued claims	64,536,000	60,054,000
Ending balance	111,685,000	104,791,000	96,934,000
Current portion of workers' compensation administrative fees accrued	2,300,000	1,800,000
Undiscounted accrued workers' compensation costs	123,400,000	118,300,000
Time period incurred claims expected to be paid recorded as restricted cash	1 year
Excess claim funds related to ACE program	2,500,000
Restricted cash- workers' compensation	47,149,000	44,737,000
Deposits-workers' compensation	64,201,000	52,320,000
Worksite Employees [Member]
Company-Sponsored 401 (k) Plans [Abstract]
Matching contributions to the Plans	63,300,000	55,700,000	47,500,000
Corporate Plan and Worksite Employee Plan [Member]
Company-Sponsored 401 (k) Plans [Abstract]
Matching contributions to the Plans	$ 65,900,000	$ 58,100,000	$ 49,600,000
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	3 years
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	10 years
Buildings and Improvements [Member] | Minimum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	5 years
Buildings and Improvements [Member] | Maximum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	30 years
Computer Hardware and Software and Acquired Technologies [Member] | Minimum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	1 year
Computer Hardware and Software and Acquired Technologies [Member] | Maximum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	5 years
Software Development Costs [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	3 years
Furniture and Fixtures [Member] | Minimum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	5 years
Furniture and Fixtures [Member] | Maximum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	7 years
Aircraft [Member] | Minimum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	15 years
Aircraft [Member] | Maximum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	20 years