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Fair value - pension (Details 8) - Qualified Plan [Member] - Underfunded Plan [Member] - Pension Benefits - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | $ 354,384 |  | $ 333,509 |  | $ 332,667
Percentage investment in common stock of international companies | 31.00% |  | 29.00% |  |
Percentage investment in corporate bonds | 28.00% |  | 21.00% |  |
Percentage investment in common stock of large-cap U.S. companies | 19.00% |  | 20.00% |  |
Percentage investment in cash equivalents | 7.00% |  | 8.00% |  |
Percentage investment in US Government securities | 1.00% |  | 7.00% |  |
Percentage investment in other investments | 14.00% |  | 15.00% |  |
Estimated future employer contributions in next fiscal year | $ 17,700 |  |  |  |
Cash equivalents [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 3,814 |  | $ 6,347 |  |
Equity securities of U.S. companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 13,345 |  | 11,348 |  |
Equities securities of International companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 1,766 |  | 1,584 |  |
Collective and mutual funds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 239,571 | [1] | 226,107 | [2] |
Corporate debt securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 74,956 |  | 68,677 |  |
Municipal bonds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 16,839 |  | 11,002 |  |
U.S. Government securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 1,038 |  | 6,396 |  |
Total assets measured at fair value [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 351,329 |  | 331,461 |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash equivalents [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
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 | 13,345 |  | 11,348 |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equities securities of International companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 1,766 |  | 1,584 |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collective and mutual funds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 171,822 | [1] | 162,055 | [2] |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 1,038 |  | 4,352 |  |
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 | 187,971 |  | 179,339 |  |
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 | 3,814 |  | 6,347 |  |
Significant Other Observable Inputs (Level 2) | Equity securities of U.S. companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
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 | 0 |  | 0 |  |
Significant Other Observable Inputs (Level 2) | Collective and mutual funds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 67,749 | [1] | 64,052 | [2] |
Significant Other Observable Inputs (Level 2) | Corporate debt securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 74,956 |  | 68,677 |  |
Significant Other Observable Inputs (Level 2) | Municipal bonds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 16,839 |  | 11,002 |  |
Significant Other Observable Inputs (Level 2) | U.S. Government securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 2,044 |  |
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 | 163,358 |  | 152,122 |  |
Significant Unobservable Inputs (Level 3) | Cash equivalents [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Significant Unobservable Inputs (Level 3) | Equity securities of U.S. companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Significant Unobservable Inputs (Level 3) | Equities securities of International companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Significant Unobservable Inputs (Level 3) | Collective and mutual funds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 | [1] | 0 | [2] |
Significant Unobservable Inputs (Level 3) | Corporate debt securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Significant Unobservable Inputs (Level 3) | Municipal bonds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Significant Unobservable Inputs (Level 3) | U.S. Government securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Significant Unobservable Inputs (Level 3) | Total assets measured at fair value [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | $ 0 |  | $ 0 |  |
[1] | Collective and mutual funds invest approximately 31 percent in common stock of international companies, 28 percent in corporate bonds, 19 percent in common stock of large-cap U.S. companies, 7 percent in cash equivalents, 1 percent in U.S. Government securities and 14 percent in other investments.
[2] | Collective and mutual funds invest approximately 29 percent in common stock of international companies, 21 percent in corporate bonds, 20 percent in common stock of large-cap U.S. companies, 8 percent in cash equivalents, 7 percent in U.S. Government securities and 15 percent in other investments.

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