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Fair value - pension and postretirement (Details 9) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Pension Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | $ 332,667,000 |  | $ 354,363,000 |  | $ 334,844,000
Percentage investment in common stock of large-cap U.S. companies | 19.00% |  | 13.00% |  |
Percentage investment in common stock of mid-cap U.S. companies | 6.00% |  |  |  |
Percentage investment in US Government securities |  |  | 13.00% |  |
Percentage investment in corporate bonds | 16.00% |  | 23.00% |  |
Percentage investment in common stock of international companies | 29.00% |  | 33.00% |  |
Percentage investment in cash equivalents | 16.00% |  |  |  |
Percentage investment in other investments | 14.00% |  | 18.00% |  |
Estimated future employer contributions in next fiscal year | $ 0 |  |  |  |
Pension Benefits | Cash equivalents [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 8,379,000 |  | $ 5,631,000 |  |
Pension Benefits | Equity securities of U.S. companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 15,135,000 |  | 39,077,000 |  |
Pension Benefits | Equities Securities of International companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 2,332,000 |  | 5,189,000 |  |
Pension Benefits | Collective and Mutual funds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 217,968,000 | [1] | 209,852,000 | [2] |
Pension Benefits | Corporate debt securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 62,145,000 |  | 59,471,000 |  |
Pension Benefits | Municipal bonds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 11,680,000 |  | 10,462,000 |  |
Pension Benefits | U.S. Government securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 12,111,000 |  | 21,850,000 |  |
Pension Benefits | Total assets measured at fair value [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 329,750,000 |  | 351,532,000 |  |
Pension Benefits | 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 |  |
Pension Benefits | 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 | 15,135,000 |  | 39,077,000 |  |
Pension Benefits | 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 | 2,332,000 |  | 5,189,000 |  |
Pension Benefits | 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 | 154,400,000 | [1] | 132,403,000 | [2] |
Pension Benefits | 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 |  |
Pension Benefits | 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 |  |
Pension Benefits | 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 | 5,288,000 |  | 15,001,000 |  |
Pension Benefits | 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 | 177,155,000 |  | 191,670,000 |  |
Pension Benefits | 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 | 8,379,000 |  | 5,631,000 |  |
Pension Benefits | 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 |  |
Pension Benefits | 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 |  |
Pension Benefits | 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 | 63,568,000 | [1] | 77,449,000 | [2] |
Pension Benefits | 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 | 62,145,000 |  | 59,471,000 |  |
Pension Benefits | 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 | 11,680,000 |  | 10,462,000 |  |
Pension Benefits | 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 | 6,823,000 |  | 6,849,000 |  |
Pension Benefits | 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 | 152,595,000 |  | 159,862,000 |  |
Pension Benefits | 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 |  |
Pension Benefits | 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 |  |
Pension Benefits | 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 |  |
Pension Benefits | 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] |
Pension Benefits | 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 |  |
Pension Benefits | 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 |  |
Pension Benefits | 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 |  |
Pension Benefits | 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 |  |
Other Postretirement Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | $ 82,593,000 |  | $ 87,586,000 |  | $ 84,543,000
Percentage investment in common stock of large-cap U.S. companies | 19.00% |  | 54.00% |  |
Percentage investment in US Government securities | 22.00% |  | 11.00% |  |
Percentage investment in corporate bonds | 36.00% |  | 10.00% |  |
Percentage investment in other investments | 13.00% |  | 15.00% |  |
Percentage investment in mortgage backed securities | 10.00% |  | 10.00% |  |
Estimated future employer contributions in next fiscal year | $ 800,000 |  |  |  |
Other Postretirement Benefits | Cash equivalents [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 3,261,000 |  | $ 2,097,000 |  |
Other Postretirement Benefits | Equity securities of U.S. companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 2,274,000 |  | 2,614,000 |  |
Other Postretirement Benefits | Equities Securities of International companies [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 9,000 |  | 25,000 |  |
Other Postretirement Benefits | Insurance investment contract |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 77,044,000 | [3] | 82,846,000 | [4] |
Other Postretirement Benefits | Total assets measured at fair value [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 82,588,000 |  | 87,582,000 |  |
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 2,283,000 |  | 2,639,000 |  |
Other Postretirement Benefits | 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 |  |
Other Postretirement Benefits | 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 | 2,274,000 |  | 2,614,000 |  |
Other Postretirement Benefits | 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 | 9,000 |  | 25,000 |  |
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance investment contract |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 | [3] | 0 | [4] |
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 80,305,000 |  | 84,943,000 |  |
Other Postretirement Benefits | 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 | 3,261,000 |  | 2,097,000 |  |
Other Postretirement Benefits | 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 |  |
Other Postretirement Benefits | 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 |  |
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Insurance investment contract |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 77,044,000 | [3] | 82,846,000 | [4] |
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Other Postretirement Benefits | 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 |  |
Other Postretirement Benefits | 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 |  |
Other Postretirement Benefits | 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 |  |
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Insurance investment contract |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | $ 0 | [3] | $ 0 | [4] |
[1] | Collective and mutual funds invest approximately 19 percent in common stock of large-cap U.S. companies, 6 percent in common stock of mid-cap U.S. companies, 16 percent in corporate bonds, 29 percent in common stock of international companies, 16 percent in cash equivalents and 14 percent in other investments.
[2] | Collective and mutual funds invest approximately 13 percent in common stock of large-cap U.S. companies, 13 percent in U.S. Government securities, 23 percent in corporate bonds, 33 percent in common stock of international companies and 18 percent in other investments.
[3] | The insurance contract invests approximately 19 percent in common stock of large-cap U.S. companies, 22 percent in U.S. Government securities, 10 percent in mortgage-backed securities, 36 percent in corporate bonds and 13 percent in other investments.
[4] | The insurance contract invests approximately 54 percent in common stock of large-cap U.S. companies, 11 percent in U.S. Government securities, 10 percent in mortgage-backed securities, 10 percent in corporate bonds and 15 percent in other investments.

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