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Accounting Policies (Details) | 3 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Oct. 01, 2010 USD ($) | Sep. 30, 2010 USD ($)
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues(1) | $ 966,756,000 | $ 925,126,000 | $ 922,295,000 | $ 1,014,372,000 | $ 826,494,000 | $ 795,513,000 | $ 795,552,000 | $ 882,664,000 | $ 3,828,549,000 | [1] | $ 3,300,223,000 | [1] | $ 2,941,347,000 | [1] |  |  |  |  |
Segment Reporting [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of reportable segments (integer) |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Land |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,215,000 | $ 6,215,000 |  |  |
Buildings and Improvements, Gross |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 112,308,000 | 95,615,000 |  |  |
Computer hardware and software |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 115,259,000 | 105,060,000 |  |  |
Software development costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 71,332,000 | 60,568,000 |  |  |
Property, Plant and Equipment, Gross |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 350,808,000 | 310,349,000 |  |  |
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (233,595,000) | (214,690,000) |  |  |
Furniture and Fixtures, Gross |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 45,694,000 | 42,891,000 |  |  |
Property, Plant and Equipment, Net |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 117,213,000 | 95,659,000 |  |  |
Amortization of capitalized computer software costs |  |  |  |  |  |  |  |  | $ 6,000,000 |  | 4,100,000 |  | 3,000,000 |  |  |  |  |  |
Unamortized computer software costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 19,600,000 | 14,900,000 |  |  |
Goodwill and Intangible Assets Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill, Impairment Loss |  |  |  |  |  |  |  |  | $ 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 |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6,000,000 |  |  |  |
Amount which Plan Costs were less than the net premiums paid and owed |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (6,300,000) |  |  |  |
Portion of insurance costs that is less than the agreed upon surplus included in accrued health insurance costs a cnt liab |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (15,300,000) |  |  |  |
Premiums owed to United |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (15,200,000) |  |  |  |
Benefits Costs Incurred Related to Run-off |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (1,300,000) | 1,200,000 | $ 5,100,000 |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,000,000 |
Reduction in accrued workers' compensation costs for changes in estimated losses |  |  |  |  |  |  |  |  | $ 18,800,000 |  | $ 16,300,000 |  | 10,900,000 |  |  |  |  |  |
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths) |  |  |  |  |  |  |  |  | 2.60% |  | 1.60% |  |  |  |  |  |  |  |
Incurred but not paid workers' compensation liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  |  |  | 207,630,000 |  |  |  | 183,928,000 | $ 207,630,000 |  | $ 183,928,000 |  |  |  |  |  |  |  |
Accrued claims |  |  |  |  |  |  |  |  | 72,066,000 |  | 68,194,000 |  |  |  |  |  |  |  |
Present value discount |  |  |  |  |  |  |  |  | (7,829,000) |  | (4,308,000) |  |  |  |  |  |  |  |
Paid claims |  |  |  |  |  |  |  |  | (42,228,000) |  | (40,184,000) |  |  |  |  |  |  |  |
Ending balance | 229,639,000 |  |  |  | 207,630,000 |  |  |  | 229,639,000 |  | 207,630,000 |  | 183,928,000 |  |  |  |  |  |
Current portion of accrued claims |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 42,227,000 | 41,137,000 |  |  |
Long-term portion of accrued claims |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 187,412,000 | 166,493,000 |  |  |
Ending Balance | $ 229,639,000 |  |  | $ 207,630,000 | $ 207,630,000 |  |  | $ 183,928,000 | 207,630,000 |  | 183,928,000 |  | $ 183,928,000 |  | 229,639,000 | 207,630,000 | $ 183,928,000 |  |
Current portion of workers' compensation administrative fees accrued |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,600,000 | 1,800,000 |  |  |
Undiscounted accrued workers' compensation costs |  |  |  |  |  |  |  |  | $ 247,400,000 |  | $ 219,900,000 |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 19,400,000 |  |  |  |  |  |  |  |  |  |
Restricted Cash and Cash Equivalents, Current |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 42,227,000 | 41,137,000 |  |  |
Deposits - workers' compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 166,474,000 | $ 154,215,000 |  |  |
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% |  |  |  |  |  |  |  |  |  |
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% |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 10,300,000 |  | $ 8,700,000 |  | $ 8,000,000 |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 165,500,000 |  | 129,000,000 |  | 108,300,000 |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |
Northeast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues(1) |  |  |  |  |  |  |  |  | $ 996,541,000 |  | 854,629,000 |  | 750,748,000 |  |  |  |  |  |
Southeast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues(1) |  |  |  |  |  |  |  |  | 447,584,000 |  | 379,874,000 |  | 318,185,000 |  |  |  |  |  |
Central [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues(1) |  |  |  |  |  |  |  |  | 637,779,000 |  | 543,486,000 |  | 467,297,000 |  |  |  |  |  |
Southwest [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues(1) |  |  |  |  |  |  |  |  | 895,243,000 |  | 767,207,000 |  | 689,334,000 |  |  |  |  |  |
West [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues(1) |  |  |  |  |  |  |  |  | 797,942,000 |  | 702,619,000 |  | 664,308,000 |  |  |  |  |  |
Other Revenues [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues(1) |  |  |  |  |  |  |  |  | $ 53,460,000 |  | $ 52,408,000 |  | $ 51,475,000 |  |  |  |  |  |
[1] | (1) Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Year ended December 31,(in thousands)201820172016 Gross billings$23,830,731$20,173,812$17,932,857Less: WSEE payroll cost20,002,18216,873,58914,991,510Revenues$3,828,549$3,300,223$2,941,347

X
- Definition Benefits costs incurred for changes in estimated run-off of claims. + References No definition available. + Details
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- Definition Long lived, depreciable assets that are used in the company's information systems operations. + References No definition available. + Details
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- Definition The decrease or increase in workers' compensation claims cost estimates for changes in the actuarial assumptions resulting from changes in actual claims experience and other trends. + References No definition available. + Details
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- Definition Carrying value of amounts transferred to third party workers' compensation insurance providers for security purposes that are expected to be returned or applied towards payment after one year or beyond the operating cycle, if longer. + References No definition available. + Details
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- Definition The number of days in advance of the beginning of a reporting quarter that United Healthcare establishes cash funding rates. + References No definition available. + Details
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- Definition The number of stock-based employee compensation plans the entity has in place at period end. + References No definition available. + Details
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- Definition Percentage the entity matches of eligible corporate employees' contributions. + References No definition available. + Details
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- Definition Minimum percentage of employees' eligible compensation matched. + References No definition available. + Details
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- Definition Maximum percentage of employees' eligible compensation matched. + References No definition available. + Details
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- Definition The percentage of grants issued to directors that are vested on the grant date for annual award grants. + References No definition available. + Details
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- Definition The portion of insurance costs that is less than the agreed upon surplus included in accrued health insurance costs a current liability, as of the balance sheet date. + References No definition available. + Details
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- Definition The premiums owed to United which is included in accrued health insurance costs, a current liability in the Company's Consolidated Balance Sheet. + References No definition available. + Details
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- Definition Carrying amount as of the balance sheet date for portion of health insurance contract to be utilized over longer than one year. + References No definition available. + Details
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- Definition Total carrying amount as of the balance sheet date for health insurance contract. + References No definition available. + Details
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- Definition Represents the amount required to be held on deposit with United, which is determined as approximately one day of claims funding activity. + References No definition available. + Details
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- Definition Return of excess claim funds related to the ACE Program recognized as long-term asset on the cosolidated balance sheet. + References No definition available. + Details
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- Definition Capitalized costs to develop software for sale or licensing, or for long-term internal use. + References No definition available. + Details
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- Definition Time period estimate of incurred claim costs to be paid included in long-term liabilities. + References No definition available. + Details
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- Definition Time period estimate of incurred claim costs to be paid included in short-term liabilities. + References No definition available. + Details
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- Definition Time period incurred claims expected to be paid included in deposits. + References No definition available. + Details
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- Definition The amount of time a workers' compensation claim is expected to be paid recorded as restricted cash, in years. + References No definition available. + Details
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