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Accounting Policies (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Oct. 01, 2010
Health Insurance Costs [Abstract] |  |  |  |  |  |  |  |
Number of days in advance of the beginning of a reporting quarter United establishes cash funding rates |  |  |  | 90 days |  |  |  |
Required accumulated cash surplus | $ 9,000 |  |  | $ 9,000 |  | $ 9,000 |  |
Required deposit equal to approximately one day of claims funding activity | 5,100 |  |  | 5,100 |  |  |  |
Amount which plan costs were less than the net premiums paid and owed | 18,900 |  |  | 18,900 |  |  |  |
Prepaid health insurance, current | 9,900 |  |  | 9,900 |  |  |  |
Premiums owed to United | 28,500 |  |  | 28,500 |  |  |  |
Benefits costs incurred (reduced) related to run-off | (1,300) |  | $ (1,000) | (1,300) | $ (1,000) |  |  |
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
Decrease Increase in accrued workers' compensation costs for changes in estimated losses |  |  |  | $ (13,400) | $ (11,000) |  |  |
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths) |  |  |  | 2.50% | 1.50% |  |  |
Incurred but not paid workers compensation liabilities [Abstract] |  |  |  |  |  |  |  |
Beginning balance, January 1, |  |  |  | $ 207,630 | $ 183,928 |  |  |
Accrued claims |  |  |  | 54,744 | 52,133 |  |  |
Present value discount |  |  |  | (5,372) | (3,048) |  |  |
Paid claims |  |  |  | (31,645) | (29,574) |  |  |
Ending balance | 225,357 |  | 203,439 | 225,357 | 203,439 |  |  |
Current portion of accrued claims | 42,258 |  | 41,748 | 42,258 | 41,748 |  |  |
Long-term portion of accrued claims | 183,099 |  | 161,691 | 183,099 | 161,691 | 166,493 |  |
Workers compensation administrative fees accrued | 3,500 |  |  | 3,500 |  |  |  |
Undiscounted accrued workers' compensation costs |  |  |  | $ 241,300 | 215,000 |  |  |
Time period incurred claims expected to be paid recorded as restricted cash |  |  |  | 1 year |  |  |  |
Return Of Excess Claim Funds |  |  |  | $ 19,400 | 22,700 |  |  |
Time period incurred claims expected to be paid, included in deposits, a long-term asset |  |  |  | Greater than 1 year |  |  |  |
Restricted cash | 42,257 |  | 41,748 | $ 42,257 | 41,748 | 41,137 | $ 42,637 |
Deposits workers compensation | 152,268 |  | 142,821 | $ 152,268 | 142,821 | $ 154,215 | $ 143,938 |
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 |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 925,126 | [1] | 795,513 | $ 2,861,793 | 2,473,729 |  |  |
Northeast [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 237,076 |  | 204,318 | 748,717 | 644,703 |  |  |
Southeast [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 109,552 |  | 92,646 | 333,004 | 283,501 |  |  |
Central [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 155,184 |  | 131,978 | 476,277 | 405,854 |  |  |
Southwest [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 217,946 |  | 184,572 | 664,731 | 573,773 |  |  |
West [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | 192,011 |  | 169,321 | 599,617 | 527,217 |  |  |
Other Revenues [Member] |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |
Revenue from Contract with Customer, Excluding Assessed Tax | $ 13,357 |  | $ 12,678 | $ 39,447 | $ 38,681 |  |  |
[1] | ____________________________________(1) Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Three Months Ended September 30, Nine Months Ended September 30,(in thousands)20182017 20182017 Gross billings$5,810,779$4,898,333 $17,284,477$14,655,545Less: WSEE payroll cost4,885,6534,102,820 14,422,68412,181,816Revenues$925,126$795,513 $2,861,793$2,473,729

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