Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 01, 2010	Sep. 30, 2010
Description of Business [Abstract]
Number of years transition to comprehensive business performance solutions provider	28 years
Segment Reporting [Abstract]
Number of reportable segments	1
Property, Plant and Equipment [Abstract]
Amortization of capitalized computer software costs	$ 4,100,000	$ 3,600,000	$ 2,800,000
Unamortized computer software costs	6,000,000	7,200,000
Property and Equipment, Estimated Useful Lives [Abstract]
Asset Impairment Charges	1,200,000
Health Insurance Costs [Abstract]
Number of days cash in advance of beginning of reporting quarter United establishes cash funding rates	90 days
Prepaid health insurance	9,000,000	9,000,000
Required deposit for health care costs	3,500,000
Amount which Plan Costs were less than the net premiums paid and owed	23,500,000
Prepaid health insurance, current	14,500,000
Premiums owed to United	15,000,000
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	2,900,000	9,300,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	1.00%	0.80%
Incurred but not paid workers' compensation liabilities
Beginning balance	120,833,000	111,685,000
Accrued claims	55,971,000	42,900,000
Present value discount	(1,998,000)	(1,169,000)
Paid claims	(38,718,000)	(32,583,000)
Ending balance	136,088,000	120,833,000	111,685,000
Current portion of accrued claims	44,040,000	51,928,000
Long-term portion of accrued claims	92,048,000	68,905,000
Ending Balance	136,088,000	120,833,000	111,685,000
Current portion of workers' compensation administrative fees accrued	1,600,000	1,000,000
Undiscounted accrued workers' compensation costs	145,800,000	131,200,000
Time period incurred claims expected to be paid recorded as restricted cash	1 year
Time period incurred claims expected to be paid, included in deposits, a long-term asset	Greater than 1 year
Additional claim funds paid related to ACE Program	7,200,000
Restricted Cash and Cash Equivalents, Current	44,040,000	51,928,000
Deposits - workers' compensation	113,934,000	81,878,000
Time period estimate of incurred claim costs to be paid included in short term liabilities	1 year
Time period incurred claims expected to be paid, included in long-term liabilities	Greater than 1 year
Stock-Based Compensation [Abstract]
Number of stock-based employee compensation plans	2
Vesting period for Initial grants to new members of the Board of Directors	3 years
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 annual grants issued to directors that are vested (in hundredths)	100.00%
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	78,400,000	71,700,000	63,300,000
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, minimum (in hundredths)	0.00%
Percentage of eligible compensation matched, maximum (in hundredths)	6.00%
Corporate Plan and Worksite Employee Plan [Member]
Company-Sponsored 401 (k) Plans [Abstract]
Matching contributions to the Plans	$ 81,500,000	$ 74,700,000	$ 65,900,000
Minimum [Member]
Other Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	3 years
Maximum [Member]
Other 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] | Minimum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	3 years
Software Development Costs [Member] | Maximum [Member]
Property and Equipment, Estimated Useful Lives [Abstract]
Estimated useful lives of property and equipment	5 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