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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | $ 363,673 | $ 316,515 | $ 249,700
Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 363,075 | 316,987 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 327,427 | 296,966 |
Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 1,615 | 1,832 |
Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 980 | 0 |
Global Asset Allocation Fund [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 41,309 | 37,878 |
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 | 37,878 |
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 | 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 | 0 |
Extended Duration Fixed Income [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 146,837 | 131,457 |
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 | 131,457 |
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 | 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 | 0 |
Short-term Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 4,939 | 23,722 |
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 | 4,939 | 23,722 |
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 | 1,615 | 1,832 |
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 | 1,615 | 1,832 |
Deposit Administration Contracts [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 | 6,554 | 25,554 |
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 | 4,939 | 23,722 |
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 | 1,615 | 1,832 |
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 |
Liability Hedging Assets [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 153,391 | 157,011 |
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 | 151,776 | 155,179 |
Liability Hedging Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 1,615 | 1,832 |
Liability Hedging Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
International equity [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 67,989 | 48,836 |
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 | 48,836 |
International equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
International equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
US Global Equity [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 66,353 | 55,073 |
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 | 55,073 |
US Global Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
US Global Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 | 0 |
Global Equity [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 134,342 | 103,909 |
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 | 134,342 | 103,909 |
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 | 16,305 | 15,466 |
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 | 1,096 | 1,615 |
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 | 460 | 1,108 |
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 | 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 |  |
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 |  |
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 |  |
Other Private Assets [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 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 |  |
Other Private Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 |  |
Other Private Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 980 |  |
Total Private Assets [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 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 |  |
Total Private Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 0 |  |
Total Private Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 980 |  |
Return Seeking Assets [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 209,684 | 159,976 |
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 | 175,651 | 141,787 |
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 | 980 | 0 |
Alternative Investments [Member] | Fair value, recurring [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets | 33,053 | 18,189 |
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 |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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