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Fair value - pension (Details 9) - Qualified Plan - Underfunded Plan - Pension Benefits - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | $ 383,834 | $ 365,264 | $ 307,809
Defined Benefit Plan, Cash and Cash Equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage | 8.00% | 6.00% |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | $ 7,841 | $ 26,166 |
Defined Benefit Plan, Equity Securities, US [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | $ 12,844 | $ 14,457 |
Defined Benefit Plan, Equity Securities, Non-US [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage | 24.00% | 29.00% |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | $ 1,727 | $ 938 |
Collective and mutual funds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | $ 233,185 | $ 219,800 |
Corporate debt securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage | 36.00% | 9.00% |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | $ 92,809 | $ 80,768 |
Municipal bonds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | $ 10,126 | $ 11,828 |
U.S. Government securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage | 5.00% | 18.00% |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | $ 13,872 | $ 9,378 |
Total assets measured at fair value [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of net plan assets at end of year | $ 383,834 | $ 363,335 |
Defined Benefit Plan, Equity Securities, US, Large Cap [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage | 18.00% | 21.00% |
Other Investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage | 9.00% | 17.00% |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Defined Benefit Plan, Equity Securities, US [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | $ 12,844 | $ 14,457 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collective and mutual funds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | 177,397 | 160,906 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | 11,177 | 7,296 |
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 | 201,418 | 182,659 |
Significant Other Observable Inputs (Level 2) | Defined Benefit Plan, Cash and Cash Equivalents [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | 7,841 | 26,166 |
Significant Other Observable Inputs (Level 2) | Defined Benefit Plan, Equity Securities, Non-US [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | 1,727 | 938 |
Significant Other Observable Inputs (Level 2) | Collective and mutual funds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | 55,788 | 58,894 |
Significant Other Observable Inputs (Level 2) | Corporate debt securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | 92,809 | 80,768 |
Significant Other Observable Inputs (Level 2) | Municipal bonds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | 10,126 | 11,828 |
Significant Other Observable Inputs (Level 2) | U.S. Government securities [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | 2,695 | 2,082 |
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 | 170,986 | $ 180,676 |
Fair Value Measured at Net Asset Value Per Share | Collective and mutual funds [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount | $ 11,430 |  |

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