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Pension and other postretirement benefits - Pension Plan Asset Allocation (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Actual plan asset allocations | 100.00% | 100.00%
Fair value of assets | $ 303,367 | $ 276,741
Level 1 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | 303,367 | 276,741
Level 2 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | 0 | 0
Level 3 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | $ 0 | $ 0
U.S. Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Target allocation percentage of assets | 29.00% |
Actual plan asset allocations | 29.00% | 34.00%
Fair value of assets | $ 86,513 | $ 93,773
U.S. Equities [Member] | Level 1 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | 86,513 | 93,773
U.S. Equities [Member] | Level 2 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | 0 | 0
U.S. Equities [Member] | Level 3 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | $ 0 | $ 0
International Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Target allocation percentage of assets | 26.00% |
Actual plan asset allocations | 26.00% | 22.00%
Fair value of assets | $ 79,120 | $ 61,453
International Equities [Member] | Level 1 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | 79,120 | 61,453
International Equities [Member] | Level 2 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | 0 | 0
International Equities [Member] | Level 3 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | $ 0 | $ 0
Fixed Income Securities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Target allocation percentage of assets | 45.00% |
Actual plan asset allocations | 45.00% | 44.00%
Fair value of assets | $ 137,734 | $ 121,515
Fixed Income Securities [Member] | Level 1 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | 137,734 | 121,515
Fixed Income Securities [Member] | Level 2 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | 0 | 0
Fixed Income Securities [Member] | Level 3 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair value of assets | $ 0 | $ 0

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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 Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + 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 (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + 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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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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