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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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-Publisher FASB
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