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Retirement plans - Asset allocation (Details)	Dec. 31, 2018	Dec. 31, 2017
Employee Benefits [Abstract]
Equity securities	20.00%	46.00%
Debt securities	57.00%	29.00%
Cash and other short-term securities	23.00%	25.00%
Total	100.00%	100.00%

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- Definition

Cash And Cash Equivalents, Amount Contributed To Fair Value Of Plan Assets, Allocation Percentage

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- Definition

Debt Instruments, Amount Contributed To Fair Value Of Plan Assets, Allocation Percentage

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- Definition

Employee Benefits [Abstract]

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- Definition

Equity Instruments, Amount Contributed To Fair Value Of Plan Assets, Allocation Percentage

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- Definition

Plan Assets At Fair Value, Allocation Percentage

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