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Pension and other postretirement benefits - Pension Plan Asset Allocation (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2015 | Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Actual Plan Asset Allocations | 100.00% | 100.00%
Fair Value of Plan assets | $ 280,862 | $ 305,520
U.S. Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Target Allocation Percentage Of Assets | 31.00% |
Actual Plan Asset Allocations | 30.00% | 42.00%
Fair Value of Plan assets | $ 86,723 | $ 79,148
International Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Target Allocation Percentage Of Assets | 21.00% |
Actual Plan Asset Allocations | 20.00% | 18.00%
Fair Value of Plan assets | $ 54,769 | $ 33,720
Fixed Income Securities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Target Allocation Percentage Of Assets | 48.00% |
Actual Plan Asset Allocations | 50.00% | 40.00%
Fair Value of Plan assets | $ 139,370 | $ 76,204
Plan receivable [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets |  | 116,448
Level 1 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 280,862 | 189,072
Level 1 [Member] | U.S. Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 86,723 | 79,148
Level 1 [Member] | International Equities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 54,769 | 33,720
Level 1 [Member] | Fixed Income Securities [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 139,370 | 76,204
Level 1 [Member] | Plan receivable [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets |  | 0
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 2 [Member] | Plan receivable [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets |  | 0
Level 3 [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets | 0 | 116,448
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
Level 3 [Member] | Plan receivable [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |
Fair Value of Plan assets |  | $ 116,448

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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 Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + 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=49178875&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=58740216&loc=d3e1928-114920 + Details
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- Definition Target allocation of investments of this type to total plan assets presented on a weighted average basis. + 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)(i) -URI http://asc.fasb.org/extlink&oid=58740216&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)(i) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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