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Regulatory assets and liabilities (Details) (USD $) | Dec. 31, 2012 | Dec. 31, 2011
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Regulatory assets and liabilities |  |  |  |
Regulatory assets: | $ 351,194,000 |  | $ 359,009,000 |
Regulatory liabilities: | 450,482,000 |  | 460,229,000 |
Net regulatory position | (99,288,000) |  | (101,220,000) |
Regulatory assets not earning a rate of return | 215,600,000 |  | 216,400,000 |
Plant removal and decommissioning costs (c) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 296,037,000 | [1] | 289,972,000 | [1]
Deferred income taxes |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 82,077,000 | [2] | 84,963,000 | [2]
Natural gas costs refundable through rate adjustments (d) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 35,328,000 | [3] | 45,064,000 | [3]
Taxes refundable to customers (c) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 24,212,000 | [1] | 31,837,000 | [1]
Other (c) (d) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Regulatory liabilities: | 12,828,000 | [1],[3] | 8,393,000 | [1],[3]
Pension and postretirement benefits (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | (e) | [4] |  |
Regulatory assets: | 166,477,000 | [4] | 171,492,000 | [4]
Deferred income taxes |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | ** | [2] |  |
Regulatory assets: | 121,781,000 | [2] | 119,189,000 | [2]
Manufactured gas plant sites remediation (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Up to 5 years | [5] |  |
Regulatory assets: | 15,828,000 | [6] | 8,150,000 | [6]
Plant costs (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Over plant lives | [5] |  |
Regulatory assets: | 10,348,000 | [6] | 10,256,000 | [6]
Long-term debt refinancing costs (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Up to 25 years | [5] |  |
Regulatory assets: | 9,144,000 | [6] | 10,112,000 | [6]
Taxes recoverable from customers (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | 0 | [5] |  |
Regulatory assets: | 9,078,000 | [6] | 12,433,000 | [6]
Costs related to identifying generation development (a) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Up to 14 years | [5] |  |
Regulatory assets: | 5,773,000 | [6] | 9,817,000 | [6]
Other (a) (b) |  |  |  |
Regulatory assets and liabilities |  |  |  |
Estimated recovery period | Largely within 1 year | [5] |  |
Regulatory assets: | $ 12,765,000 | [6],[7] | $ 17,560,000 | [6],[7]
[1] | Included in other liabilities on the Consolidated Balance Sheets.
[2] | Represents deferred income taxes related to regulatory assets and liabilities. The deferred income tax assets are not earning a rate of return.
[3] | Included in other accrued liabilities on the Consolidated Balance Sheets.
[4] | Recovered as expense is incurred.
[5] | Estimated recovery period for regulatory assets currently being recovered in rates charged to customers.
[6] | Included in deferred charges and other assets on the Consolidated Balance Sheets.
[7] | Included in prepayments and other current assets on the Consolidated Balance Sheets.

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- Definition Estimated recovery period of regulatory assets. + References No definition available. + Details
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- Definition The amount of regulatory assets, excluding deferred income taxes, that were not earning a rate of return. + References No definition available. + Details
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- Definition The net amount of all regulatory assets less all regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 9, 10 -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -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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