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Fair value - other postretirement (Details 10) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage investment in common stock of small-cap companies | 10.00% | 10.00% |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | $ 94,587 | $ 82,516 | $ 88,739
Percentage investment in corporate bonds | 50.00% | 51.00% |
Percentage investment in common stock of large-cap U.S. companies | 25.00% | 23.00% |
Percentage Investment in US Government Securities | 7.00% | 7.00% |
Percentage investment in common stock of small-cap companies | 7.00% | 7.00% |
Percentage investment in other investments | 11.00% | 12.00% |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Cash equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | $ 4,017 | $ 3,866 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Equity securities of U.S. companies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 2,073 | 1,767 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Equities securities of International companies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 1 | 2 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Insurance investment contract [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 88,496 | 76,881 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Total assets measured at fair value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 94,587 | 82,516 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities of U.S. companies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 2,073 | 1,767 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance investment contract [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 10 | 1 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Total assets measured at fair value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 2,083 | 1,768 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Cash equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 4,017 | 3,866 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Equities securities of International companies [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 1 | 2 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Insurance investment contract [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | 88,486 | 76,880 |
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Total assets measured at fair value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | $ 92,504 | $ 80,748 |

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