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Retirement benefits - Assets Held In Retirement Benefit Accounts (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 1,618,703 | $ 1,369,701 | $ 1,271,474
Other benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 193,995 | 174,251 | $ 170,687
Fair value measurements on a recurring basis | Pension benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 1,615,000 | 1,366,000 |
NAV of plan assets | 315,000 | 269,000 |
Cash, receivables and payables, net | 4,000 | 4,000 |
Fair value of plan assets, net | 1,619,000 | 1,370,000 |
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 | 1,079,000 | 877,000 |
Fair value measurements on a recurring basis | Pension benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 568,000 | 692,000 |
NAV of plan assets | 76,000 | 56,000 |
Fair value measurements on a recurring basis | Pension benefits | U.S. Equity Index Fund |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 435,000 | 129,000 |
Fair value measurements on a recurring basis | Pension benefits | Non-U.S. Equity Funds |  |  |
Fair value measurements on a recurring basis |  |  |
NAV of plan assets | $ 76,000 | $ 56,000 |
Redemption frequency, daily | 32.00% | 31.00% |
Redemption frequency, monthly | 68.00% | 31.00% |
Redemption frequency, quarterly |  | 38.00% |
Fair value measurements on a recurring basis | Pension benefits | Non-U.S. Equity Funds | Minimum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 5 days | 0 days |
Fair value measurements on a recurring basis | 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 | Pension benefits | Fixed Income Funds, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 500,000 | $ 456,000 |
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 | 297,000 | 276,000 |
NAV of plan assets | $ 203,000 | $ 180,000 |
Redemption notice period (in days) | 15 days | 10 days |
Fair value measurements on a recurring basis | Pension benefits | Cash Equivalents |  |  |
Fair value measurements on a recurring basis |  |  |
NAV of plan assets | $ 36,000 | $ 33,000 |
Fair value measurements on a recurring basis | Pension benefits | Cash Equivalents | Minimum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 0 days | 0 days |
Fair value measurements on a recurring basis | Pension benefits | Cash Equivalents | Maximum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 1 day | 1 day |
Fair value measurements on a recurring basis | Other benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 194,000 | $ 174,000 |
NAV of plan assets | 21,000 | 19,000 |
Cash, receivables and payables, net | 0 | 0 |
Fair value of plan assets, net | 194,000 | 174,000 |
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 | 139,000 | 120,000 |
Fair value measurements on a recurring basis | Other benefits | Equity securities |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 75,000 | 94,000 |
NAV of plan assets | 12,000 | 9,000 |
Fair value measurements on a recurring basis | Other benefits | U.S. Equity Index Fund |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 52,000 | 17,000 |
Fair value measurements on a recurring basis | Other benefits | Non-U.S. Equity Funds |  |  |
Fair value measurements on a recurring basis |  |  |
NAV of plan assets | $ 12,000 | $ 9,000 |
Redemption frequency, daily | 26.00% |  |
Redemption frequency, monthly | 74.00% | 57.00% |
Redemption frequency, quarterly |  | 42.00% |
Fair value measurements on a recurring basis | Other benefits | Non-U.S. Equity Funds | Minimum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 5 days | 10 days |
Fair value measurements on a recurring basis | 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 | Other benefits | Fixed Income Funds, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 50,000 | $ 48,000 |
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 | 46,000 | 44,000 |
NAV of plan assets | $ 4,000 | $ 4,000 |
Redemption notice period (in days) | 15 days | 10 days |
Fair value measurements on a recurring basis | Other benefits | Cash Equivalents |  |  |
Fair value measurements on a recurring basis |  |  |
NAV of plan assets | $ 5,000 | $ 6,000 |
Fair value measurements on a recurring basis | Other benefits | Cash Equivalents | Minimum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 0 days | 0 days |
Fair value measurements on a recurring basis | Other benefits | Cash Equivalents | Maximum |  |  |
Fair value measurements on a recurring basis |  |  |
Redemption notice period (in days) | 1 day | 1 day |
Fair value measurements on a recurring basis | Level 1 | Pension benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 1,084,000 | $ 905,000 |
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 | 1,003,000 | 821,000 |
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 | 568,000 | 692,000 |
Fair value measurements on a recurring basis | Level 1 | Pension benefits | U.S. Equity Index Fund |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 435,000 | 129,000 |
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Fixed Income Funds, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 81,000 | 84,000 |
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 | 81,000 | 84,000 |
Fair value measurements on a recurring basis | Level 1 | Other benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 170,000 | 153,000 |
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 | 127,000 | 111,000 |
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 | 75,000 | 94,000 |
Fair value measurements on a recurring basis | Level 1 | Other benefits | U.S. Equity Index Fund |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 52,000 | 17,000 |
Fair value measurements on a recurring basis | Level 1 | Other benefits | Fixed Income Funds, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 43,000 | 42,000 |
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 | 43,000 | 42,000 |
Fair value measurements on a recurring basis | Level 2 | Pension benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 216,000 | 192,000 |
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 | U.S. Equity Index Fund |  |  |
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 Funds, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 216,000 | 192,000 |
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 | 216,000 | 192,000 |
Fair value measurements on a recurring basis | Level 2 | Other benefits |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 3,000 | 2,000 |
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 | U.S. Equity Index Fund |  |  |
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 Funds, Measured at Fair Value and Net Asset Value |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | 3,000 | 2,000 |
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 | 3,000 | 2,000 |
Fair value measurements on a recurring basis | Level 3 | Pension benefits |  |  |
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 | U.S. Equity Index Fund |  |  |
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 Funds, 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 | 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 |  |  |
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 | U.S. Equity Index Fund |  |  |
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 Funds, 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 | Total fixed income investments |  |  |
Fair value measurements on a recurring basis |  |  |
Fair value of plan assets | $ 0 | $ 0 |

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