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Employee benefit plans - Schedule of Pension Plan Weighted-Average Asset Allocations (Details) - Pension Benefits | Dec. 31, 2017 | Dec. 31, 2016
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Defined Benefit Plan Disclosure |  |
Weighted-average asset allocations | 100.00% | 100.00%
Cash and cash equivalents |  |
Defined Benefit Plan Disclosure |  |
Weighted-average asset allocations | 4.10% | 4.20%
Common equity securities |  |
Defined Benefit Plan Disclosure |  |
Weighted-average asset allocations | 57.40% | 58.00%
Corporate bonds |  |
Defined Benefit Plan Disclosure |  |
Weighted-average asset allocations | 27.60% | 26.20%
U.S. Government securities |  |
Defined Benefit Plan Disclosure |  |
Weighted-average asset allocations | 10.90% | 11.60%

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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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