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Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Percentage of customer revenue | 43.00% | 43.00% | 45.00%
Accounts receivable recorded investment past due days | 90 |  |
Self insurance retention liability | $ 3.0 | $ 2.0 | $ 2.0
Self insurance retention liability additional deductible | 5.0 |  |
Self insurance retention Workers' compensation | 1.0 |  |
Letters of credit outstanding, amount | 39.6 |  |
Cumulative effect on retained earnings | 0.3 |  |
State of Nebraska [Member] |  |  |
Workers' compensation insurance bonds | 26.6 |  |
Workers' Compensation Insurance Carrier [Member] |  |  |
Workers' compensation insurance bonds | 6.9 |  |
Minimum |  |  |
Self insurance retention liability additional deductible applicable range | 5.0 |  |
Maximum |  |  |
Self insurance retention liability additional deductible applicable range | $ 10.0 |  |

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