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Regulatory assets and liabilities (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Regulatory assets and liabilities |  |  |
Regulatory assets: |  | $ 300,963 | $ 307,320
Regulatory liabilities: |  | 519,298 | 241,852
Net regulatory position |  | (218,335) | 65,468
Regulatory assets not earning a rate of return |  | 269,100 | 255,400
Plant removal and decommissioning costs [Member} |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1] | 176,190 | 176,972
Taxes recoverable from/refundable to customers [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1] | 279,668 | 11,010
Pension and postretirement benefits [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1] | 11,056 | 9,099
Natural gas costs refundable through rate adjustments [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [2] | 28,514 | 25,580
Other regulatory assets/liabilities [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1],[2] | $ 23,870 | 19,191
Pension and postretirement benefits [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [3] | (e) |
Regulatory assets: | [4] | $ 163,896 | 176,025
Taxes recoverable from/refundable to customers [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Over plant lives |
Regulatory assets: | [4] | $ 12,073 | 28,278
Manufactured gas plant sites remediation [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | - |
Regulatory assets: | [4] | $ 18,213 | 18,259
Asset retirement obligation costs [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Over plant lives |
Regulatory assets: | [4] | $ 56,078 | 42,580
Natural gas costs recoverable through rate adjustments [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 1 year |
Regulatory assets: | [6] | $ 14,465 | 2,242
Long-term debt refinancing costs [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 20 years |
Regulatory assets: | [4] | $ 5,563 | 6,248
Costs related to identifying generation development [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 9 years |
Regulatory assets: | [4] | $ 2,960 | 3,407
Other regulatory assets/liabilities [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 20 years |
Regulatory assets: | [4],[6] | $ 27,715 | $ 30,281
Other noncurrent assets [Member] | Stranded costs [Member] |  |  |
Regulatory assets and liabilities |  |  |
Deferred tax liabilities, regulatory assets |  | 15,500 |
Other noncurrent liabilities [Member] | Stranded costs [Member] |  |  |
Regulatory assets and liabilities |  |  |
Deferred tax liabilities, regulatory assets |  | $ 270,000 |
[1] | Included in deferred credits and other liabilities - other 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 Amount of deferred tax liability attributable to taxable temporary differences from regulatory assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=96869850&loc=d3e28680-109314 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319 + Details
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