Accounting Policies (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Oct. 01, 2010 USD ($)	Sep. 30, 2010 USD ($)
Segment Reporting [Abstract]
Number of reportable segments (integer)		1
Property, Plant and Equipment [Line Items]
Amortization of capitalized computer software costs		$ 3,300	$ 4,100	$ 3,600
Unamortized computer software costs					$ 7,100	$ 6,000
Asset impairment charge due to change in office consolidation plans	$ 1,200
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	9,000
Required deposit for health care costs					3,500
Amount which Plan Costs were less than the net premiums paid and owed					2,200
Portion of insurance costs that is less than the agreed upon surplus included in accrued health insurance costs a cnt liab					(6,800)
Premiums owed to United					3,100
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence								$ 1,000
Company's maximum aggregate economic burden for claims in excess of $1 million per policy year							$ 5,000
Reduction in accrued workers' compensation costs for changes in estimated losses		$ 1,300	$ 2,900
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)		1.00%	1.00%
Incurred but not paid workers' compensation liabilities
Beginning balance		$ 136,088	$ 120,833
Accrued claims		67,559	55,971
Present value discount		(3,095)	(1,998)
Paid claims		(38,368)	(38,718)
Ending balance	136,088	162,184	136,088	120,833
Current portion of accrued claims					37,438	44,040
Long-term portion of accrued claims					124,746	92,048
Ending Balance	$ 136,088	136,088	120,833	120,833	162,184	136,088
Current portion of workers' compensation administrative fees accrued					1,600
Undiscounted accrued workers' compensation costs		$ 172,300	145,800
Time period incurred claims expected to be paid recorded as restricted cash		1 year
Return Of Excess Claim Funds		$ 5,300
Time period incurred claims expected to be paid, included in deposits, a long-term asset		Greater than 1 year
Restricted Cash and Cash Equivalents, Current					37,418	44,040
Deposits - workers' compensation					$ 136,462	$ 113,934
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%
New Accounting Pronouncements [Abstract]
Prior Period Reclassification Adjustment			6,300
Minimum [Member]
Other Intangible Assets [Line Items]
Other Intangible Assets, useful life		3 years
Maximum [Member]
Other Intangible Assets [Line Items]
Other Intangible Assets, useful life		10 years
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%
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		$ 95,300	78,400	71,700
Corporate Plan and Worksite Employee Plan [Member]
Company-Sponsored 401 (k) Plans [Abstract]
Matching contributions to the Plans		$ 98,700	$ 81,500	$ 74,700
Buildings and Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		5 years
Buildings and Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		30 years
Computer Hardware and Software and Acquired Technologies [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		2 years
Computer Hardware and Software and Acquired Technologies [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		5 years
Software Development Costs [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		3 years
Software Development Costs [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		5 years
Furniture, Fixtures and Other [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		5 years
Furniture, Fixtures and Other [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		7 years
Aircraft [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		15 years
Aircraft [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment		20 years