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Retirement benefits Retirement benefits - Assets Held In Retirement Benefit Accounts (Details) - USD ($)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets $ 1,369,701,000 $ 1,271,474,000 $ 1,266,060,000
Other benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets 174,251,000 170,687,000 $ 180,332,000
Fair value measurements on a recurring basis | Pension benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets 1,366,000,000 1,268,000,000  
NAV of plan assets 269,000,000 249,000,000  
Cash, receivables and payables, net 4,000,000 3,000,000  
Fair value of plan assets, net 1,370,000,000 1,271,000,000  
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 877,000,000 805,000,000  
Fair value measurements on a recurring basis | Pension benefits | Equity securities      
Fair value measurements on a recurring basis      
Fair value of plan assets 692,000,000 640,000,000  
NAV of plan assets 56,000,000 46,000,000  
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 129,000,000 119,000,000  
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 $ 56,000,000 $ 46,000,000  
Redemption frequency, daily 31.00% 24.00%  
Redemption frequency, monthly 31.00% 29.00%  
Redemption frequency, quarterly 38.00% 47.00%  
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) 0 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 $ 456,000,000 $ 425,000,000  
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 276,000,000 260,000,000  
NAV of plan assets $ 180,000,000 $ 165,000,000  
Redemption notice period (in days) 10 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 $ 33,000,000 $ 38,000,000  
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 $ 174,000,000 $ 171,000,000  
NAV of plan assets 19,000,000 18,000,000  
Cash, receivables and payables, net 0 0  
Fair value of plan assets, net 174,000,000 171,000,000  
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 120,000,000 118,000,000  
Fair value measurements on a recurring basis | Other benefits | Equity securities      
Fair value measurements on a recurring basis      
Fair value of plan assets 94,000,000 92,000,000  
NAV of plan assets 9,000,000 9,000,000  
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 17,000,000 17,000,000  
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 $ 9,000,000 $ 9,000,000  
Redemption frequency, monthly 57.00% 54.00%  
Redemption frequency, quarterly 42.00% 46.00%  
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) 10 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 $ 48,000,000 $ 48,000,000  
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 44,000,000 44,000,000  
NAV of plan assets $ 4,000,000 $ 4,000,000  
Redemption notice period (in days) 10 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 $ 6,000,000 $ 5,000,000  
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 $ 905,000,000 $ 844,000,000  
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 821,000,000 759,000,000  
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 692,000,000 640,000,000  
NAV of plan assets 0 0  
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 129,000,000 119,000,000  
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 84,000,000 85,000,000  
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 84,000,000 85,000,000  
NAV of plan assets 0 0  
Fair value measurements on a recurring basis | Level 1 | Pension benefits | Cash Equivalents      
Fair value measurements on a recurring basis      
NAV of plan assets 0 0  
Fair value measurements on a recurring basis | Level 1 | Other benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets 153,000,000 151,000,000  
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 111,000,000 109,000,000  
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 94,000,000 92,000,000  
NAV of plan assets 0 0  
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 17,000,000 17,000,000  
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 42,000,000 42,000,000  
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 42,000,000 42,000,000  
NAV of plan assets 0 0  
Fair value measurements on a recurring basis | Level 1 | Other benefits | Cash Equivalents      
Fair value measurements on a recurring basis      
NAV of plan assets 0 0  
Fair value measurements on a recurring basis | Level 2 | Pension benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets 192,000,000 175,000,000  
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  
NAV 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 192,000,000 175,000,000  
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 192,000,000 175,000,000  
NAV of plan assets 0 0  
Fair value measurements on a recurring basis | Level 2 | Pension benefits | Cash Equivalents      
Fair value measurements on a recurring basis      
NAV of plan assets 0 0  
Fair value measurements on a recurring basis | Level 2 | Other benefits      
Fair value measurements on a recurring basis      
Fair value of plan assets 2,000,000 2,000,000  
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  
NAV 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 2,000,000 2,000,000  
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 2,000,000 2,000,000  
NAV of plan assets 0 0  
Fair value measurements on a recurring basis | Level 2 | Other benefits | Cash Equivalents      
Fair value measurements on a recurring basis      
NAV of plan assets 0 0  
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  
NAV 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  
NAV of plan assets 0 0  
Fair value measurements on a recurring basis | Level 3 | Pension benefits | Cash Equivalents      
Fair value measurements on a recurring basis      
NAV 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  
NAV 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  
NAV of plan assets 0 0  
Fair value measurements on a recurring basis | Level 3 | Other benefits | Cash Equivalents      
Fair value measurements on a recurring basis      
NAV of plan assets $ 0 $ 0