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Expected rate of return (Details 6) - Underfunded Plan [Member] - Qualified Plan [Member] | Dec. 31, 2017
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Minimum [Member] | Equity securities [Member] | Pension Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, plan assets, target allocation, percentage | 40.00%
Minimum [Member] | Equity securities [Member] | Other Postretirement Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, plan assets, target allocation, percentage | 20.00%
Minimum [Member] | Fixed Income Securities [Member] | Pension Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, plan assets, target allocation, percentage | 50.00%
Minimum [Member] | Fixed Income Securities [Member] | Other Postretirement Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, plan assets, target allocation, percentage | 75.00%
Maximum [Member] | Equity securities [Member] | Pension Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, plan assets, target allocation, percentage | 50.00%
Maximum [Member] | Equity securities [Member] | Other Postretirement Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, plan assets, target allocation, percentage | 25.00%
Maximum [Member] | Fixed Income Securities [Member] | Pension Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, plan assets, target allocation, percentage | 60.00%
Maximum [Member] | Fixed Income Securities [Member] | Other Postretirement Benefits |
Defined Benefit Plan Disclosure [Line Items] |
Defined benefit plan, plan assets, target allocation, percentage | 80.00%

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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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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