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Available-for-sale securities (Details) - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015
Available-for-sale securities [Line Items] |  |  |
Investments used to satisfy obligations under nonqualified benefit plans | $ 69,100 | $ 67,800 | $ 67,500
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans | 1,600 | 2,000 |
Available-for-sale securities [Abstract] |  |  |
Cost | 10,467 | 10,129 | 10,443
Gross unrealized gains | 46 | 67 | 19
Gross unrealized losses | (14) | (22) | (55)
Fair value | 10,499 | 10,174 | 10,407
Mortgage backed securities [Member] |  |  |
Available-for-sale securities [Abstract] |  |  |
Cost | 10,467 | 7,792 | 9,128
Gross unrealized gains | 46 | 58 | 19
Gross unrealized losses | (14) | (18) | (49)
Fair value | $ 10,499 | 7,832 | 9,098
US Treasury securities [Member] |  |  |
Available-for-sale securities [Abstract] |  |  |
Cost |  | 2,337 | 1,315
Gross unrealized gains |  | 9 | 0
Gross unrealized losses |  | (4) | (6)
Fair value |  | $ 2,342 | $ 1,309

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- Definition Net unrealized gain (loss) of investments used to satisfy obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees. + References No definition available. + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562 + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 + Details
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