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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