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Retirement plans - Asset allocation (Details) | Dec. 31, 2017 | Dec. 31, 2016
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Employee Benefits [Abstract] |  |
Equity securities | 46.00% | 49.00%
Debt securities | 29.00% | 27.00%
Cash and other short-term securities | 25.00% | 24.00%
Total | 100.00% | 100.00%

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