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401(k) Plan (Details) - USD ($) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Sep. 30, 2016
Retirement Benefits [Abstract] |  |  |  |  |
Employee contribution, percentage of annual salary |  | 100.00% |  |  |
Employee contribution, maximum allowable contribution |  | $ 55,000 |  |  |
Service period for eligibility (in years) | 1 month |  | 1 year |  |
Number of hours of service required in 12 months for plan eligibility (in hours) |  | 1000 hours |  |  |
Employer matching contribution percent |  | 100.00% |  |  |
Percent of employees' gross pay |  | 4.00% |  |  |
Term for employees to become fully vested (in years) |  | 6 years |  |  |
Profit sharing contribution, percent |  | 7.00% |  |  |
Company contributions to plan |  | $ 5,910,000 |  | $ 6,433,000 | $ 7,600,000

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