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Retirement benefits - Assets Held In Retirement Benefit Accounts (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Pension benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 1,479,067 | $ 1,618,703 | $ 1,369,701
Other benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 173,693 | 193,995 | $ 174,251
Fair value measurements on a recurring basis | Pension benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 1,479,000 | 1,619,000 |
Fair value measurements on a recurring basis | Pension benefits | Total |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 1,474,000 | 1,615,000 |
Fair value measurements on a recurring basis | Pension benefits | Equity Securities, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 920,000 | 1,079,000 |
Fair value measurements on a recurring basis | Pension benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 507,000 | 568,000 |
Fair value measurements on a recurring basis | Pension benefits | Equity index and exchange-traded funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 348,000 | $ 435,000 |
Fair value measurements on a recurring basis | Pension benefits | Non-U.S. Equity Funds |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption frequency, daily | 32.00% | 32.00% |
Redemption frequency, monthly | 68.00% | 68.00% |
Fair value measurements on a recurring basis | Pension benefits | Fixed income securities and public mutual funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 310,000 | $ 297,000 |
Fair value measurements on a recurring basis | Pension benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 518,000 | 500,000 |
Fair value measurements on a recurring basis | Pension benefits | Cash, receivables and payables, net |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 5,000 | 4,000 |
Fair value measurements on a recurring basis | Other benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 174,000 | 194,000 |
Fair value measurements on a recurring basis | Other benefits | Total |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 173,000 | 194,000 |
Fair value measurements on a recurring basis | Other benefits | Equity Securities, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 117,000 | 139,000 |
Fair value measurements on a recurring basis | Other benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 65,000 | 75,000 |
Fair value measurements on a recurring basis | Other benefits | Equity index and exchange-traded funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 42,000 | $ 52,000 |
Fair value measurements on a recurring basis | Other benefits | Non-U.S. Equity Funds |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption frequency, daily | 27.00% | 26.00% |
Redemption frequency, monthly | 73.00% | 74.00% |
Fair value measurements on a recurring basis | Other benefits | Fixed income securities and public mutual funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 47,000 | $ 46,000 |
Fair value measurements on a recurring basis | Other benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 51,000 | 50,000 |
Fair value measurements on a recurring basis | Other benefits | Cash, receivables and payables, net |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 1,000 | 0 |
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Total |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 978,000 | 1,084,000 |
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Equity Securities, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 855,000 | 1,003,000 |
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 507,000 | 568,000 |
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Equity index and exchange-traded funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 348,000 | 435,000 |
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Fixed income securities and public mutual funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 123,000 | 81,000 |
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 123,000 | 81,000 |
Fair value measurements on a recurring basis | Level 1 | Other benefits | Total |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 152,000 | 170,000 |
Fair value measurements on a recurring basis | Level 1 | Other benefits | Equity Securities, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 107,000 | 127,000 |
Fair value measurements on a recurring basis | Level 1 | Other benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 65,000 | 75,000 |
Fair value measurements on a recurring basis | Level 1 | Other benefits | Equity index and exchange-traded funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 42,000 | 52,000 |
Fair value measurements on a recurring basis | Level 1 | Other benefits | Fixed income securities and public mutual funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 45,000 | 43,000 |
Fair value measurements on a recurring basis | Level 1 | Other benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 45,000 | 43,000 |
Fair value measurements on a recurring basis | Level 2 | Pension benefits | Total |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 187,000 | 216,000 |
Fair value measurements on a recurring basis | Level 2 | Pension benefits | Equity Securities, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 2 | Pension benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 2 | Pension benefits | Equity index and exchange-traded funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 2 | Pension benefits | Fixed income securities and public mutual funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 187,000 | 216,000 |
Fair value measurements on a recurring basis | Level 2 | Pension benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 187,000 | 216,000 |
Fair value measurements on a recurring basis | Level 2 | Other benefits | Total |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 2,000 | 3,000 |
Fair value measurements on a recurring basis | Level 2 | Other benefits | Equity Securities, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 2 | Other benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 2 | Other benefits | Equity index and exchange-traded funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 2 | Other benefits | Fixed income securities and public mutual funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 2,000 | 3,000 |
Fair value measurements on a recurring basis | Level 2 | Other benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 2,000 | 3,000 |
Fair value measurements on a recurring basis | Level 3 | Pension benefits | Total |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Pension benefits | Equity Securities, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Pension benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Pension benefits | Equity index and exchange-traded funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Pension benefits | Fixed income securities and public mutual funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Pension benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Other benefits | Total |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Other benefits | Equity Securities, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Other benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Other benefits | Equity index and exchange-traded funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Other benefits | Fixed income securities and public mutual funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Level 3 | Other benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 0 | 0 |
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 309,000 | 315,000 |
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 65,000 | 76,000 |
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Non-U.S. Equity Funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 65,000 | $ 76,000 |
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Non-U.S. Equity Funds | Minimum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 5 days | 5 days |
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Non-U.S. Equity Funds | Maximum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 30 days | 30 days |
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 208,000 | $ 203,000 |
Redemption notice period (in days) | 15 days | 15 days |
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Cash equivalents at NAV |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 36,000 | $ 36,000 |
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Cash equivalents at NAV | Minimum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 0 days | 0 days |
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Cash equivalents at NAV | Maximum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 1 day | 1 day |
Fair value measurements on a recurring basis | Net Asset Value | Other benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 19,000 | $ 21,000 |
Fair value measurements on a recurring basis | Net Asset Value | Other benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 10,000 | 12,000 |
Fair value measurements on a recurring basis | Net Asset Value | Other benefits | Non-U.S. Equity Funds |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 10,000 | $ 12,000 |
Fair value measurements on a recurring basis | Net Asset Value | Other benefits | Non-U.S. Equity Funds | Minimum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 5 days | 5 days |
Fair value measurements on a recurring basis | Net Asset Value | Other benefits | Non-U.S. Equity Funds | Maximum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 30 days | 30 days |
Fair value measurements on a recurring basis | Net Asset Value | Other benefits | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 4,000 | $ 4,000 |
Redemption notice period (in days) | 15 days | 10 days |
Fair value measurements on a recurring basis | Net Asset Value | Other benefits | Cash equivalents at NAV |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 5,000 | $ 5,000 |
Fair value measurements on a recurring basis | Net Asset Value | Other benefits | Cash equivalents at NAV | Minimum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 0 days | 0 days |
Fair value measurements on a recurring basis | Net Asset Value | Other benefits | Cash equivalents at NAV | Maximum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 1 day | 1 day |

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