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Fair value - other postretirement plans (Details 11) (USD $) | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
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Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Percentage investment in common stock of large-cap U.S. companies | 29.00% |  | 32.00% |  |
Other Postretirement Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Percentage investment in common stock of large-cap U.S. companies | 51.00% |  |  |  |
Percentage investment in mortgage backed securities | 10.00% |  | 12.00% |  |
Percentage investment in common stock of large-cap U.S. companies |  |  | 49.00% |  |
Percentage investment in US Treasuries | 15.00% |  | 15.00% |  |
Percentage investment in corporate bonds | 11.00% |  | 11.00% |  |
Percentage investment in other investments | 13.00% |  | 13.00% |  |
Fair value of net plan assets at end of year | $ 74,361,000 |  | $ 68,085,000 |  | $ 70,610,000
Estimated future employer contributions in next fiscal year | 2,300,000 |  |  |  |
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 70,841,000 |  | 65,666,000 |  |
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 3,520,000 |  | 2,419,000 |  |
Other Postretirement Benefits | Cash equivalents |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 3,044,000 |  | 1,895,000 |  |
Other Postretirement Benefits | Cash equivalents | Significant Other Observable Inputs (Level 2) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 1,991,000 |  | 1,836,000 |  |
Other Postretirement Benefits | Cash equivalents | Significant Unobservable Inputs (Level 3) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Other Postretirement Benefits | Cash equivalents | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 1,053,000 |  | 59,000 |  |
Other Postretirement Benefits | Equity securities of U.S. companies |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 2,207,000 |  | 2,098,000 |  |
Other Postretirement Benefits | Equity securities of U.S. companies | Significant Other Observable Inputs (Level 2) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Other Postretirement Benefits | Equity securities of U.S. companies | Significant Unobservable Inputs (Level 3) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Other Postretirement Benefits | Equity securities of U.S. companies | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 2,207,000 |  | 2,098,000 |  |
Other Postretirement Benefits | Equity securities of International companies |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 260,000 |  | 262,000 |  |
Other Postretirement Benefits | Equity securities of International companies | Significant Other Observable Inputs (Level 2) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Other Postretirement Benefits | Equity securities of International companies | Significant Unobservable Inputs (Level 3) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 |  | 0 |  |
Other Postretirement Benefits | Equity securities of International companies | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 260,000 |  | 262,000 |  |
Other Postretirement Benefits | Insurance investment contract |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 68,850,000 | [1] | 63,830,000 | [2] |
Other Postretirement Benefits | Insurance investment contract | Significant Other Observable Inputs (Level 2) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 68,850,000 | [1] | 63,830,000 | [2] |
Other Postretirement Benefits | Insurance investment contract | Significant Unobservable Inputs (Level 3) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | 0 | [1] | 0 | [2] |
Other Postretirement Benefits | Insurance investment contract | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Fair value of net plan assets at end of year | $ 0 | [1] | $ 0 | [2] |
[1] | The insurance investment contract invests approximately 51 percent in common stock of large-cap U.S. companies, 15 percent in U.S. Treasuries, 10 percent in mortgage-backed securities, 11 percent in corporate bonds and 13 percent in other investments.
[2] | The insurance investment contract invests approximately 49 percent in common stock of large-cap U.S. companies, 15 percent in U.S. Treasuries, 12 percent in mortgage-backed securities, 11 percent in corporate bonds and 13 percent in other investments.

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- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + 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 (g) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph g -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=6867990&loc=d3e12355-114930 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 87 -Paragraph 49 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph d(iv)(b)(i) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 + Details
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