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Note 7 - Pensions and Other Postretirement Benefits (Details Textual) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement | $ 4,000,000 | $ 0 | $ 3,800,000
Unrecognized Actuarial Gain (Loss) in Excess of Benefit Obligation, Percentage | 10.00% |  |
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components | $ 271,000 |  |
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation | 1,600,000 |  |
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components | 233,000 |  |
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation | $ 1,400,000 |  |
Minimum [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 7.00% |  |
Minimum [Member] | Fixed Income Securities [Member] |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 70.00% |  |
Minimum [Member] | Alternative Investments [Member] |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 0.00% |  |
Minimum [Member] | Cash and Cash Equivalents [Member] |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 0.00% |  |
Maximum [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 27.00% |  |
Maximum [Member] | Fixed Income Securities [Member] |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 90.00% |  |
Maximum [Member] | Alternative Investments [Member] |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 6.00% |  |
Maximum [Member] | Cash and Cash Equivalents [Member] |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage | 10.00% |  |
One Percentage Point Change in Assumed Rate of Return [Member] |  |  |
Defined Benefit Plan Effect On Pension Expense | $ 783,000 |  |
One Percentage Point Increase in Discount Rate [Member] |  |  |
Defined Benefit Plan Effect On Pension Expense | 1,200,000 |  |
One Percentage Point Decrease in Discount Rate [Member] |  |  |
Defined Benefit Plan Effect On Pension Expense | 1,400,000 |  |
Pension Plan [Member] |  |  |
Defined Contribution Plan, Employer Discretionary Contribution Amount | 1,900,000 | $ 1,600,000 | $ 1,600,000
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year | $ 2,000,000 |  |
Postretirement Health Coverage [Member] | Retirees Age65 And Over [Member] |  |  |
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate | 0.00% |  |
Postretirement Health Coverage [Member] | Retirees Under Age65 [Member] |  |  |
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate | 5.00% |  |

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- Definition Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate. + 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 (m) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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