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Available-for-sale securities (Details) - USD ($) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014 | Dec. 31, 2014
Available-for-sale securities [Line Items] |  |  |  |  |
Investments used to satisfy obligations under nonqualified benefit plans | $ 68,200,000 | $ 64,400,000 | $ 68,200,000 | $ 64,400,000 | $ 65,800,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans | 400,000 | 1,100,000 | 2,400,000 | 2,000,000 |
Available-for-sale securities [Abstract] |  |  |  |  |
Cost | 10,399,000 | 10,055,000 | 10,399,000 | 10,055,000 | 10,168,000
Gross unrealized gains | 51,000 | 121,000 | 51,000 | 121,000 | 60,000
Gross unrealized losses | (28,000) | (5,000) | (28,000) | (5,000) | (37,000)
Fair value | 10,422,000 | 10,171,000 | 10,422,000 | 10,171,000 | 10,191,000
Mortgage backed securities [Member] |  |  |  |  |
Available-for-sale securities [Abstract] |  |  |  |  |
Cost | 8,072,000 | 7,989,000 | 8,072,000 | 7,989,000 | 6,594,000
Gross unrealized gains | 29,000 | 91,000 | 29,000 | 91,000 | 60,000
Gross unrealized losses | (28,000) | (5,000) | (28,000) | (5,000) | (18,000)
Fair value | 8,073,000 | 8,075,000 | 8,073,000 | 8,075,000 | 6,636,000
US Treasury securities [Member] |  |  |  |  |
Available-for-sale securities [Abstract] |  |  |  |  |
Cost | 2,327,000 | 2,066,000 | 2,327,000 | 2,066,000 | 3,574,000
Gross unrealized gains | 22,000 | 30,000 | 22,000 | 30,000 | 0
Gross unrealized losses | 0 | 0 | 0 | 0 | (19,000)
Fair value | $ 2,349,000 | $ 2,096,000 | $ 2,349,000 | $ 2,096,000 | $ 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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- 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=27724398&loc=d3e27161-111563 + Details
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