Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 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	$ 4,100,000	$ 3,000,000	$ 3,300,000
Unamortized computer software costs				$ 14,900,000	$ 10,400,000
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill, Impairment Loss	0	0	0
Goodwill, Gross				21,156,000	21,156,000
Goodwill, Impaired, Accumulated Impairment Loss				(8,470,000)	(8,470,000)
Goodwill				12,686,000	12,686,000
Impairment of Intangible Assets, Finite-lived	$ 0	0	0
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				5,100,000
Amount which Plan Costs were less than the net premiums paid and owed				12,100,000
Portion of insurance costs that is less than the agreed upon surplus included in accrued health insurance costs a cnt liab				3,100,000
Premiums owed to United				(21,900,000)
Benefits Costs Incurred Related to Run-off				1,200,000
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	$ 16,300,000	$ 10,900,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)	1.60%	1.00%
Incurred but not paid workers' compensation liabilities
Beginning balance	$ 183,928,000	$ 162,184,000
Accrued claims	68,194,000	65,069,000
Present value discount	(4,308,000)	(2,628,000)
Paid claims	(40,184,000)	(40,697,000)
Ending balance	207,630,000	183,928,000	162,184,000
Current portion of accrued claims				41,137,000	42,637,000
Long-term portion of accrued claims				166,493,000	141,291,000
Ending Balance	183,928,000	162,184,000	$ 162,184,000	207,630,000	183,928,000
Current portion of workers' compensation administrative fees accrued				1,800,000	1,600,000
Undiscounted accrued workers' compensation costs	$ 219,900,000	$ 194,200,000
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
Return Of Excess Claim Funds	$ 22,700,000
Restricted Cash and Cash Equivalents, Current				41,137,000	42,637,000
Deposits - workers' compensation				$ 154,215,000	$ 143,938,000
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				1
Vesting period for Initial grants to new members of the Board of Directors	0 years	3 years
Vesting period for officers and other management to be eligible for restricted stock grants, minimum	3 years
Percentage of annual grants issued to directors that are vested (in hundredths)	100.00%
New Accounting Pronouncements [Abstract]
Income tax benefit recognized related to Equity Compensation after adoption of ASU 2016-09, per share | $ / shares	$ 0.16	$ 0.04
Income tax benefit recognized related to Equity Compensation after adoption of ASU 2016-09, Quantification	$ 6,800,000	$ 1,500,000
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]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage the entity matches of eligible corporate employees' contributions	100.00%	100.00%	50.00%
Percentage of eligible compensation matched, Minimum	0.00%	0.00%	0.00%
Percentage of eligible compensation matched, maximum	6.00%	6.00%	6.00%
Matching contributions to the plan	$ 8,700,000	$ 8,000,000	$ 3,400,000
Worksite Employees [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of eligible compensation matched, Minimum	0.00%
Percentage of eligible compensation matched, maximum	6.00%
Matching contributions to the plan	$ 129,000,000	$ 108,300,000	$ 95,300,000
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