Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Description of Business [Abstract]
Number of years transition to comprehensive business performance solutions provider (in years)	25
Markets Served [Abstract]
Number of reportable segments	1
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of intangible assets, minimum (in years)	3
Estimated useful lives of intangible assets, maximum (in years)	10
Intangible assets and goodwill:
Gross Carrying Amount	$ 31,984,000	$ 23,071,000
Accumulated Amortization	(3,551,000)	(1,820,000)
Net Carrying Amount	28,433,000	21,251,000
Amortization expense related to purchased intangible assets other than goodwill	1,700,000	799,000	408,000
Goodwill, beginning balance	14,327,000
Acquisitions	6,829,000
Goodwill, ending balance	21,156,000	14,327,000
Future amortization expense [Abstract]
Future amortization expense, year one	1,700,000
Future amortization expense, year two	1,700,000
Future amortization expense, year three	1,600,000
Future amortization expense, year four	1,200,000
Health Insurance Costs [Abstract]
Number of days cash in advance of beginning of reporting quarter United establishes cash funding rates (in days)	90D
Required accumulated cash surplus	9,000,000
Amount which Plan Costs were less than the net premiums paid and owed	24,000,000
Prepaid health insurance, current	15,000,000
Premiums owed to United	6,100,000
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	11,400,000	6,200,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	1.10%	1.40%
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 (in years)	3
Vesting period for officers and other management to be eligible for restricted stock grants, maximum (in years)	5
Percentage of grants issued to directors that are vested (in hundredths)	100.00%
Incurred but not paid workers' compensation liabilities
Beginning balance	96,934,000	88,450,000
Accrued claims	36,845,000	34,345,000
Present value discount	(1,513,000)	(1,675,000)
Paid claims	(27,475,000)	(24,186,000)
Ending balance	104,791,000	96,934,000	88,450,000
Current portion of accrued claims	44,737,000	41,204,000
Long-term portion of accrued claims	60,054,000	55,730,000
Ending balance	104,791,000	96,934,000	88,450,000
Current portion of workers' compensation administrative fees accrued	1,800,000	877,000
Undiscounted accrued workers' compensation costs	118,300,000	111,500,000
Time period incurred claims expected to be paid recorded as restricted cash (in years)	1Y
Excess claim funds related to ACE program	10,000,000
Restricted cash- workers' compensation	44,737,000	41,204,000
Deposits-workers' compensation	52,320,000	51,731,000
Corporate Employees [Member]
Company-Sponsored 401 (k) Plans [Abstract]
Percentage match of eligible corporate employees' contributions (in hundredths)	50.00%
Percentage of eligible compensation matched, maximum (in hundredths)	6.00%	6.00%	6.00%
Worksite Employees [Member]
Company-Sponsored 401 (k) Plans [Abstract]
Percentage of eligible compensation matched, minimum (in hundredths)	0.00%
Percentage of eligible compensation matched, maximum (in hundredths)	6.00%
Matching contributions to the Plans	55,700,000	47,500,000	45,100,000
Corporate Plan and Worksite Employee Plan [Member]
Company-Sponsored 401 (k) Plans [Abstract]
Matching contributions to the Plans	58,100,000	49,600,000	47,700,000
Goodwill [Member]
Intangible assets and goodwill:
Gross Carrying Amount	21,156,000	14,327,000
Accumulated Amortization	0	0
Net Carrying Amount	21,156,000	14,327,000
Trademarks [Member]
Intangible assets and goodwill:
Gross Carrying Amount	1,785,000	1,785,000
Accumulated Amortization	(742,000)	(568,000)
Net Carrying Amount	1,043,000	1,217,000
Customer Relationships [Member]
Intangible assets and goodwill:
Gross Carrying Amount	9,043,000	6,959,000
Accumulated Amortization	(2,809,000)	(1,252,000)
Net Carrying Amount	$ 6,234,000	$ 5,707,000
Buildings and Improvments [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Property and equipment, estimated useful lives, minimum (in years)	5
Property and equipment, estimated useful lives, maximum (in years)	30
Computer Hardware and Software and Acquired Technologies [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Property and equipment, estimated useful lives, minimum (in years)	1
Property and equipment, estimated useful lives, maximum (in years)	5
Software Development Costs [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Property and equipment, estimated useful lives, average (in years)	3
Furniture and Fixtures [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Property and equipment, estimated useful lives, minimum (in years)	5
Property and equipment, estimated useful lives, maximum (in years)	7
Aircraft [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Property and equipment, estimated useful lives, average (in years)	20
Texas [Member]
Markets Served [Abstract]
Percentage of revenues by state (in hundredths)	27.00%	28.00%	29.00%
California [Member]
Markets Served [Abstract]
Percentage of revenues by state (in hundredths)	16.00%	15.00%	15.00%