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Benefit Plans - Fair Value of Plan Assets (Details) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | ---
Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | $ 360,900 | $ 350,107 |
Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 24,260 | 10,251 |
Pension Plan | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 336,640 | 339,856 |
Pension Plan | Domestic Mutual Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 11,805 | 1,442 |
Pension Plan | Domestic Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 11,805 | 1,442 |
Pension Plan | Domestic Mutual Funds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Corporate Bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 178,412 | 188,332 |
Pension Plan | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Corporate Bonds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 178,412 | 188,332 |
Pension Plan | U.S. Government Securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 7,115 | 3,630 |
Pension Plan | U.S. Government Securities | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 6,761 | 3,133 |
Pension Plan | U.S. Government Securities | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 354 | 497 |
Pension Plan | Municipal Bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 63,492 | 61,206 |
Pension Plan | Municipal Bonds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Municipal Bonds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 63,492 | 61,206 |
Pension Plan | Foreign Bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 27,917 | 25,251 |
Pension Plan | Foreign Bonds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Foreign Bonds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 27,917 | 25,251 |
Pension Plan | Exchange traded fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 5,694 | 5,676 |
Pension Plan | Exchange traded fund | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 5,694 | 5,676 |
Pension Plan | Exchange traded fund | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Pooled Equity Accounts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 66,465 | 64,570 |
Pension Plan | Pooled Equity Accounts | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Pooled Equity Accounts | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 66,465 | 64,570 |
Other Postretirement Benefit Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 70,408 | 65,568 | $ 66,940
Other Postretirement Benefit Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 70,408 | 65,568 |
Other Postretirement Benefit Plan | Domestic Mutual Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 54,426 | 49,887 |
Other Postretirement Benefit Plan | Domestic Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 54,426 | 49,887 |
Other Postretirement Benefit Plan | International Mutual Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 15,982 | 15,681 |
Other Postretirement Benefit Plan | International Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | $ 15,982 | $ 15,681 |

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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=49178875&loc=d3e12355-114930 Reference 2: 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=58740216&loc=d3e1928-114920 + Details
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