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Retirement Plans (Quantitative Disclosures of Retirement Income Plan's Invested Assets that are Measured at Fair Value on a Recurring Basis) (Details) - Retirement Income Plan [Member] - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | $ 331,680 | $ 363,673 | $ 316,515
Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 331,834 | 363,075 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 300,725 | 327,427 |
Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 1,482 | 1,615 |
Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 980 |
Extended Duration Fixed Income [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 146,837 |
Extended Duration Fixed Income [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 146,837 |
Extended Duration Fixed Income [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 0 |
Extended Duration Fixed Income [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 0 |
Global Asset Allocation Fund [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 41,309 |
Global Asset Allocation Fund [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 41,309 |
Global Asset Allocation Fund [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 0 |
Global Asset Allocation Fund [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 0 |
International equity [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 67,989 |
International equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 67,989 |
International equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 0 |
International equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 0 |
US Global Equity [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 66,353 |
US Global Equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 66,353 |
US Global Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 0 |
US Global Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets |  | 0 |
Global Equity [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 113,409 | 134,342 |
Global Equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 113,409 | 134,342 |
Global Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Global Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Real Assets [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 16,818 | 16,305 |
Real Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Real Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Real Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Private Equity Funds [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 878 | 1,096 |
Private Equity Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Private Equity Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Private Equity Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Private Credit Funds [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 262 | 460 |
Private Credit Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Private Credit Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Private Credit Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Hedge Funds [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 7,889 | 15,192 |
Hedge Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Hedge Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Hedge Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 25,847 | 33,053 |
Alternative Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Alternative Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Alternative Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Other Private Assets [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 3,780 | 980 |
Other Private Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Other Private Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Other Private Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 980 |
Total Private Assets [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 29,627 | 34,033 |
Total Private Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Total Private Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Total Private Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 980 |
Return Seeking Assets [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 143,036 | 209,684 |
Return Seeking Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 113,409 | 175,651 |
Return Seeking Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Return Seeking Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 980 |
Fixed Income Securities [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 106,000 |  |
Fixed Income Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 106,000 |  |
Fixed Income Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 |  |
Fixed Income Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 |  |
US Treasury Securities [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 18,528 |  |
US Treasury Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 18,528 |  |
US Treasury Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 |  |
US Treasury Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 |  |
Short-term Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 62,788 | 4,939 |
Short-term Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 62,788 | 4,939 |
Short-term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Short-term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Deposit Administration Contracts [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 1,482 | 1,615 |
Deposit Administration Contracts [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Deposit Administration Contracts [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 1,482 | 1,615 |
Deposit Administration Contracts [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Liability Hedging Assets [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 124,528 | 146,837 |
Liability Hedging Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 124,528 | 146,837 |
Liability Hedging Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Liability Hedging Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Cash And Short Term Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 64,270 | 6,554 |
Cash And Short Term Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 62,788 | 4,939 |
Cash And Short Term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 1,482 | 1,615 |
Cash And Short Term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | $ 0 | $ 0 |

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