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Regulatory assets and liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
Regulatory assets and liabilities |  |  |
Regulatory assets: |  | $ 263,258 | $ 269,586
Regulatory liabilities: |  | 243,118 | 386,252
Net regulatory position |  | 20,140 | (116,666)
Regulatory assets not earning a rate of return |  | 224,700 | 229,600
Plant removal and decommissioning costs [Member} |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1] | 182,981 | 338,641
Taxes refundable to customers [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1] | 17,060 | 17,772
Natural gas costs refundable through rate adjustments [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [2] | 20,884 | 13,238
Other regulatory assets/liabilities [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1],[2] | $ 22,193 | 16,601
Pension and postretirement benefits [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [3] | (e) |
Regulatory assets: | [4] | $ 185,832 | 182,565
Taxes recoverable from customers [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Over plant lives |
Regulatory assets: | [4] | $ 27,682 | 22,910
Manufactured gas plant sites remediation [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 2 years |
Regulatory assets: | [4] | $ 18,617 | 17,548
Plant costs [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 1 year |
Regulatory assets: | [4] | $ 8,000 | 4,551
Natural gas costs recoverable through rate adjustments [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 1 year |
Regulatory assets: | [6] | $ 547 | 19,575
Long-term debt refinancing costs [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 22 years |
Regulatory assets: | [4] | $ 7,031 | 7,864
Costs related to identifying generation development [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 11 years |
Regulatory assets: | [4] | $ 3,808 | 4,165
Other regulatory assets/liabilities [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Largely within 1- 4 years |
Regulatory assets: | [4],[6] | $ 11,741 | $ 10,408
[1] | Included in other liabilities on the Consolidated Balance Sheets.
[2] | Included in other accrued liabilities on the Consolidated Balance Sheets.
[3] | Recovered as expense is incurred or cash contributions are made.
[4] | Included in deferred charges and other assets - other on the Consolidated Balance Sheets.
[5] | Estimated recovery period for regulatory assets currently being recovered in rates charged to customers.
[6] | Included in prepayments and other current assets on the Consolidated Balance Sheets.

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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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