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Benefit Plans Narrative (Details) | 12 Months Ended
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Dec. 31, 2018 USD ($) yr | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Pension Plans [Abstract] |  |  |
Number of compensation years included in benefit calculation | yr | 3 |  |
Expected future working lifetime of participants | 12 years 10 months 28 days |  |
Market gains (losses) | $ (1,388,000) | $ 200,000 |
Return on investments for past 10 years, percent | 8.49% |  |
Investments benchmark for past 10 years, percent | 8.54% |  |
Return on investments for past five years, percent | 5.00% |  |
Investments benchmark for past five years, percent | 5.16% |  |
Flexible Spending Plan [Abstract] |  |  |
Annual maximum contribution limit under Health Care Spending Account plan | $ 2,500.0 |  |
Annual maximum contribution limit under Dependent Care Spending Account plan | 5,000 |  |
Deferral Plan [Abstract] |  |  |
Employer contributions to deferral plan | $ 1,465,000 | 1,585,000 | $ 1,242,000
Fixed Income Funds [Member] |  |  |
Pension Plans [Abstract] |  |  |
Target plan asset allocations | 25.00% |  |
Bonds [Member] |  |  |
Pension Plans [Abstract] |  |  |
Actual plan asset allocations | 34.00% |  |
Cash and Cash Equivalents [Member] |  |  |
Pension Plans [Abstract] |  |  |
Actual plan asset allocations | 6.00% |  |
Equity Securities [Member] |  |  |
Pension Plans [Abstract] |  |  |
Actual plan asset allocations | 60.00% |  |
Private Corporation Assets [Member] |  |  |
Pension Plans [Abstract] |  |  |
Target plan asset allocations | 5.00% |  |
Minimum [Member] |  |  |
Pension Plans [Abstract] |  |  |
Period for performance standards | 3 years |  |
Maximum [Member] |  |  |
Pension Plans [Abstract] |  |  |
Period for performance standards | 5 years |  |
Executive Supplemental Retirement Plan [Member] |  |  |
Pension Plans [Abstract] |  |  |
Projected benefit obligation | $ 25,380,000 | 24,832,000 |
Net periodic pension cost | 2,905,000 | 2,186,000 | 1,729,000
Executives and Non-employee Directors [Member] | Deferred Compensation Arrangement with Individual, by Type of Compensation, Pension and Other Postretirement Benefits [Member] |  |  |
Special Deferral Election Plan and Deferral Election Program [Abstract] |  |  |
Deferrals by executives and non-employee directors | $ 4,244,000 | $ 4,528,000 | $ 4,250,000

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