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Employee Benefits - Pension Plan Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value Measurements, Recurring - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [1] | $ 87,074 | $ 87,613
Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [2] | 1,780 | 3,336
Fixed income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [3] | 27,738 | 27,279
Equity index mutual funds—international |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [4] | 4,016 | 3,362
Domestic individual equities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [5] | 53,540 | 53,636
Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [1] | 85,294 | 84,277
Level 1 | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [2] | 0 | 0
Level 1 | Fixed income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [3] | 27,738 | 27,279
Level 1 | Equity index mutual funds—international |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [4] | 4,016 | 3,362
Level 1 | Domestic individual equities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [5] | 53,540 | 53,636
Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [1] | 1,780 | 3,336
Level 2 | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [2] | 1,780 | 3,336
Level 2 | Fixed income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [3] | 0 | 0
Level 2 | Equity index mutual funds—international |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [4] | 0 | 0
Level 2 | Domestic individual equities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [5] | 0 | 0
Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [1] | 0 | 0
Level 3 | Cash and cash equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [2] | 0 | 0
Level 3 | Fixed income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [3] | 0 | 0
Level 3 | Equity index mutual funds—international |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [4] | 0 | 0
Level 3 | Domestic individual equities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total Assets at Fair Value | [5] | $ 0 | $ 0
[1] | Refer to Note 1 Summary of Significant Accounting Policies, Fair Value Measurements for a description of levels within the fair value hierarchy.
[2] | This asset class includes FDIC insured money market instruments.
[3] | This asset class includes a variety of fixed income mutual funds which primarily invest in investment grade rated securities. Investment managers have discretion to invest in fixed income related securities including futures, options and other derivatives. Investments may be made in currencies other than the U.S. dollar.
[4] | The sole investment within this asset class is MSCI EAFE Index iShares.
[5] | This asset class includes individual domestic equities invested in an active all-cap strategy. It may also include convertible bonds.

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