| **Retirement Plans and Other Benefits - Amounts Recognized on the Consolidated Balance Sheets (Details) - USD ($) $ in Thousands** | Dec. 31, 2017 | Dec. 31, 2016 |
|------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Amounts recognized on the Consolidated Balance Sheets**                                                                          |          |          |
| Noncurrent asset                                                                                                                   | $ 268,978     | $ 166,206     |
| Pension Benefits                                                                                                                   |          |          |
| **Amounts recognized on the Consolidated Balance Sheets**                                                                          |          |          |
| Noncurrent asset                                                                                                                   | 0             | 0             |
| Current liability                                                                                                                  | (9,859)       | (19,795)      |
| Noncurrent liability                                                                                                               | (327,300)     | (509,310)     |
| Net amount recognized                                                                                                              | (337,159)     | (529,105)     |
| Other Benefits                                                                                                                     |          |          |
| **Amounts recognized on the Consolidated Balance Sheets**                                                                          |          |          |
| Noncurrent asset                                                                                                                   | 268,978       | 166,206       |
| Current liability                                                                                                                  | 0             | 0             |
| Noncurrent liability                                                                                                               | 0             | 0             |
| Net amount recognized                                                                                                              | $ 268,978     | $ 166,206     |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans.
+ References
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| X                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as current.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as noncurrent.
+ References
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| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                  |
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