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Retirement benefits - Assets Held In Retirement Benefit Accounts (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 1,799,200 $ 1,479,067 $ 1,618,703
Other benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets 200,831 173,693 $ 193,995
Fair value measurements on a recurring basis | Pension benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets 1,799,000 1,479,000  
Fair value measurements on a recurring basis | Pension benefits | Total, excluding cash, receivables and payables, net      
Fair value measurements on a recurring basis      
Fair value of plan assets 1,795,000 1,474,000  
Fair value measurements on a recurring basis | Pension benefits | Equity investments      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 1,158,000 $ 920,000  
Fair value measurements on a recurring basis | Pension benefits | Non-U.S. Equity Funds      
Fair value measurements on a recurring basis      
Redemption frequency, daily 60.00% 32.00%  
Redemption frequency, monthly 40.00% 68.00%  
Fair value measurements on a recurring basis | Pension benefits | Fixed income securities      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 598,000 $ 518,000  
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 4,000 5,000  
Fair value measurements on a recurring basis | Other benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets 201,000 174,000  
Fair value measurements on a recurring basis | Other benefits | Total, excluding cash, receivables and payables, net      
Fair value measurements on a recurring basis      
Fair value of plan assets 201,000 173,000  
Fair value measurements on a recurring basis | Other benefits | Equity investments      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 141,000 $ 117,000  
Fair value measurements on a recurring basis | Other benefits | Non-U.S. Equity Funds      
Fair value measurements on a recurring basis      
Redemption frequency, daily 59.00% 27.00%  
Redemption frequency, monthly 41.00% 73.00%  
Fair value measurements on a recurring basis | Other benefits | Fixed income securities      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 56,000 $ 51,000  
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 0 1,000  
Fair value measurements on a recurring basis | Fair Value, Inputs, Level 1, 2 and 3 | Pension benefits | Equity securities      
Fair value measurements on a recurring basis      
Fair value of plan assets 470,000 507,000  
Fair value measurements on a recurring basis | Fair Value, Inputs, Level 1, 2 and 3 | Pension benefits | Equity index and exchange-traded funds      
Fair value measurements on a recurring basis      
Fair value of plan assets 610,000 348,000  
Fair value measurements on a recurring basis | Fair Value, Inputs, Level 1, 2 and 3 | Pension benefits | Fixed income securities      
Fair value measurements on a recurring basis      
Fair value of plan assets 353,000 310,000  
Fair value measurements on a recurring basis | Fair Value, Inputs, Level 1, 2 and 3 | Other benefits | Equity securities      
Fair value measurements on a recurring basis      
Fair value of plan assets 61,000 65,000  
Fair value measurements on a recurring basis | Fair Value, Inputs, Level 1, 2 and 3 | Other benefits | Equity index and exchange-traded funds      
Fair value measurements on a recurring basis      
Fair value of plan assets 69,000 42,000  
Fair value measurements on a recurring basis | Fair Value, Inputs, Level 1, 2 and 3 | Other benefits | Fixed income securities      
Fair value measurements on a recurring basis      
Fair value of plan assets 52,000 47,000  
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Total, excluding cash, receivables and payables, net      
Fair value measurements on a recurring basis      
Fair value of plan assets 1,203,000 978,000  
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Equity investments      
Fair value measurements on a recurring basis      
Fair value of plan assets 1,080,000 855,000  
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 470,000 507,000  
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 610,000 348,000  
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Fixed income securities      
Fair value measurements on a recurring basis      
Fair value of plan assets 123,000 123,000  
Fair value measurements on a recurring basis | Level 1 | Other benefits | Total, excluding cash, receivables and payables, net      
Fair value measurements on a recurring basis      
Fair value of plan assets 179,000 152,000  
Fair value measurements on a recurring basis | Level 1 | Other benefits | Equity investments      
Fair value measurements on a recurring basis      
Fair value of plan assets 130,000 107,000  
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 61,000 65,000  
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 69,000 42,000  
Fair value measurements on a recurring basis | Level 1 | Other benefits | Fixed income securities      
Fair value measurements on a recurring basis      
Fair value of plan assets 49,000 45,000  
Fair value measurements on a recurring basis | Level 2 | Pension benefits | Total, excluding cash, receivables and payables, net      
Fair value measurements on a recurring basis      
Fair value of plan assets 230,000 187,000  
Fair value measurements on a recurring basis | Level 2 | Pension benefits | Equity investments      
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      
Fair value measurements on a recurring basis      
Fair value of plan assets 230,000 187,000  
Fair value measurements on a recurring basis | Level 2 | Other benefits | Total, excluding cash, receivables and payables, net      
Fair value measurements on a recurring basis      
Fair value of plan assets 2,000 2,000  
Fair value measurements on a recurring basis | Level 2 | Other benefits | Equity investments      
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      
Fair value measurements on a recurring basis      
Fair value of plan assets 2,000 2,000  
Fair value measurements on a recurring basis | Level 3 | Pension benefits | Total, excluding cash, receivables and payables, net      
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 investments      
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      
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, excluding cash, receivables and payables, net      
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 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 | 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      
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 362,000 309,000  
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Equity investments      
Fair value measurements on a recurring basis      
Fair value of plan assets 78,000 65,000  
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 $ 78,000 $ 65,000  
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 | Fixed income securities      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 245,000 $ 208,000  
Redemption notice period (in days) 15 days 15 days  
Fair value measurements on a recurring basis | Net Asset Value | Pension benefits | Cash equivalents      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 39,000 $ 36,000  
Fair value measurements on a recurring basis | Net Asset Value | 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 | Net Asset Value | 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 | Net Asset Value | Other benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 19,000 $ 19,000  
Fair value measurements on a recurring basis | Net Asset Value | Other benefits | Equity investments      
Fair value measurements on a recurring basis      
Fair value of plan assets 11,000 10,000  
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 $ 11,000 $ 10,000  
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 | Fixed income securities      
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      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 4,000 $ 5,000  
Fair value measurements on a recurring basis | Net Asset Value | 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 | Net Asset Value | Other benefits | Cash equivalents | Maximum      
Fair value measurements on a recurring basis      
Redemption notice period (in days) 1 day 1 day