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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 |  |  |  |  |  |

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