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Available-for-sale securities (Details) (USD $) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012 | Dec. 31, 2012
Schedule of available-for-sale securities [Line Items] |  |  |  |  |
Investments used to satisfy obligations under nonqualified benefit plans | $ 54,000,000 | $ 46,000,000 | $ 54,000,000 | $ 46,000,000 | $ 48,900,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans | 700,000 | (2,700,000) | 5,100,000 | 2,200,000 |
Available-for-sale securities [Abstract] |  |  |  |  |
Available-for-sale securities, amortized cost basis | 47,225,000 | 47,338,000 | 47,225,000 | 47,338,000 | 47,067,000
Available-for-sale securities, gross unrealized gains |  |  | 16,842,000 | 8,874,000 | 11,835,000
Available-for-sale securities, gross unrealized losses |  |  | (56,000) | (6,000) | (3,000)
Available-for-sale securities | 64,011,000 | 56,206,000 | 64,011,000 | 56,206,000 | 58,899,000
Insurance contract [Member] |  |  |  |  |
Available-for-sale securities [Abstract] |  |  |  |  |
Available-for-sale securities, amortized cost basis | 37,270,000 | 37,250,000 | 37,270,000 | 37,250,000 | 37,250,000
Available-for-sale securities, gross unrealized gains |  |  | 16,769,000 | 8,709,000 | 11,648,000
Available-for-sale securities, gross unrealized losses |  |  | 0 | 0 | 0
Available-for-sale securities | 54,039,000 | 45,959,000 | 54,039,000 | 45,959,000 | 48,898,000
Collateralized mortgage backed securities [Member] |  |  |  |  |
Available-for-sale securities [Abstract] |  |  |  |  |
Available-for-sale securities, amortized cost basis | 8,035,000 | 8,130,000 | 8,035,000 | 8,130,000 | 8,054,000
Available-for-sale securities, gross unrealized gains |  |  | 58,000 | 128,000 | 144,000
Available-for-sale securities, gross unrealized losses |  |  | (41,000) | (5,000) | (3,000)
Available-for-sale securities | 8,052,000 | 8,253,000 | 8,052,000 | 8,253,000 | 8,195,000
US Treasury securities [Member] |  |  |  |  |
Available-for-sale securities [Abstract] |  |  |  |  |
Available-for-sale securities, amortized cost basis | 1,920,000 | 1,958,000 | 1,920,000 | 1,958,000 | 1,763,000
Available-for-sale securities, gross unrealized gains |  |  | 15,000 | 37,000 | 43,000
Available-for-sale securities, gross unrealized losses |  |  | (15,000) | (1,000) | 0
Available-for-sale securities | $ 1,920,000 | $ 1,994,000 | $ 1,920,000 | $ 1,994,000 | $ 1,806,000

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- Definition 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 Net unrealized gain (loss) of investments used to satify 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=28360136&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=6871852&loc=d3e26610-111562 + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities. + 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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- Definition Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities. + 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 (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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