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Pension Benefit and Retirement Health and Life Insurance Benefits (Fair Value of Net Assets by Asset Category) (Details) - Pension Benefits - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension assets | $ 180,056 | $ 171,778 | $ 171,007
Pooled separate accounts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension assets | 4,610 | 7,587 |
Fixed income bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension assets | 162,934 | 111,070 |
Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension assets | 6,223 | 44,054 |
Guaranteed deposit account |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension assets | $ 6,289 | $ 9,067 |

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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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