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Pension and other postretirement benefits - Pension Plan Asset Allocation (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Actual Plan Asset Allocations | 100.00% | 100.00%
Fair Value of Plan assets | $ 276,741 | $ 280,862
U.S. Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Target Allocation Percentage Of Assets | 33.00% |
Actual Plan Asset Allocations | 34.00% | 30.00%
Fair Value of Plan assets | $ 93,773 | $ 86,723
International Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Target Allocation Percentage Of Assets | 22.00% |
Actual Plan Asset Allocations | 22.00% | 20.00%
Fair Value of Plan assets | $ 61,453 | $ 54,769
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 | 44.00% | 50.00%
Fair Value of Plan assets | $ 121,515 | $ 139,370
Level 1 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 276,741 | 280,862
Level 1 [Member] | U.S. Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 93,773 | 86,723
Level 1 [Member] | International Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 61,453 | 54,769
Level 1 [Member] | Fixed Income Securities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 121,515 | 139,370
Level 2 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 0 | 0
Level 2 [Member] | U.S. Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 0 | 0
Level 2 [Member] | International Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 0 | 0
Level 2 [Member] | Fixed Income Securities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 0 | 0
Level 3 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 0 | 0
Level 3 [Member] | U.S. Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 0 | 0
Level 3 [Member] | International Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 0 | 0
Level 3 [Member] | Fixed Income Securities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | $ 0 | $ 0

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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 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930 Reference 2: 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=65897934&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6749337 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6520858 + Details
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