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Available-for-sale securities (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014 | Dec. 31, 2014
Available-for-sale securities [Line Items] |  |  |  |  |
Investments used to satisfy obligations under nonqualified benefit plans | $ 66,500,000 | $ 63,600,000 | $ 66,500,000 | $ 63,600,000 | $ 65,800,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans | (1,700,000) | (800,000) | 700,000 | 1,200,000 |
Available-for-sale securities [Abstract] |  |  |  |  |
Cost | 10,167,000 | 10,206,000 | 10,167,000 | 10,206,000 | 10,168,000
Gross unrealized gains | 33,000 | 79,000 | 33,000 | 79,000 | 60,000
Gross unrealized losses | (22,000) | (10,000) | (22,000) | (10,000) | (37,000)
Fair value | 10,178,000 | 10,275,000 | 10,178,000 | 10,275,000 | 10,191,000
Mortgage backed securities [Member] |  |  |  |  |
Available-for-sale securities [Abstract] |  |  |  |  |
Cost | 7,843,000 | 7,838,000 | 7,843,000 | 7,838,000 | 6,594,000
Gross unrealized gains | 29,000 | 71,000 | 29,000 | 71,000 | 60,000
Gross unrealized losses | (18,000) | (8,000) | (18,000) | (8,000) | (18,000)
Fair value | 7,854,000 | 7,901,000 | 7,854,000 | 7,901,000 | 6,636,000
US Treasury securities [Member] |  |  |  |  |
Available-for-sale securities [Abstract] |  |  |  |  |
Cost | 2,324,000 | 2,368,000 | 2,324,000 | 2,368,000 | 3,574,000
Gross unrealized gains | 4,000 | 8,000 | 4,000 | 8,000 | 0
Gross unrealized losses | (4,000) | (2,000) | (4,000) | (2,000) | (19,000)
Fair value | $ 2,324,000 | $ 2,374,000 | $ 2,324,000 | $ 2,374,000 | $ 3,555,000

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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=51813488&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=27724398&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=49167202&loc=d3e26610-111562 + Details
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