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Regulatory assets and liabilities (Tables) | 12 Months Ended
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Dec. 31, 2017
Regulatory Assets and Liabilities Disclosure [Abstract] |
Regulatory assets | The following table summarizes the individual components of unamortized regulatory assets and liabilities as of December 31:
| Estimated Recovery Period | * | 2017 |  | 2016 |
|  |  | (In thousands)
Regulatory assets: |  |  |  |
Pension and postretirement benefits (a) | (e) |  | $ | 163,896 |  | $ | 176,025 |
Taxes recoverable from customers (a) | Over plant lives |  | 12,073 |  | 28,278 |
Manufactured gas plant sites remediation (a) | - |  | 18,213 |  | 18,259 |
Asset retirement obligations (a) | Over plant lives |  | 56,078 |  | 42,580 |
Natural gas costs recoverable through rate adjustments (b) | Up to 1 year |  | 14,465 |  | 2,242 |
Long-term debt refinancing costs (a) | Up to 20 years |  | 5,563 |  | 6,248 |
Costs related to identifying generation development (a) | Up to 9 years |  | 2,960 |  | 3,407 |
Other (a) (b) | Up to 20 years |  | 27,715 |  | 30,281 |
Total regulatory assets |  |  | 300,963 |  | 307,320 |
Regulatory liabilities: |  |  |  |
Plant removal and decommissioning costs (c) |  |  | 176,190 |  | 176,972 |
Taxes refundable to customers (c) |  |  | 279,668 |  | 11,010 |
Pension and postretirement benefits (c) |  |  | 11,056 |  | 9,099 |
Natural gas costs refundable through rate adjustments (d) |  |  | 28,514 |  | 25,580 |
Other (c) (d) |  |  | 23,870 |  | 19,191 |
Total regulatory liabilities |  |  | 519,298 |  | 241,852 |
Net regulatory position |  |  | $ | (218,335 | ) | $ | 65,468 |
* | Estimated recovery period for regulatory assets currently being recovered in rates charged to customers.
(a) | Included in deferred charges and other assets - other on the Consolidated Balance Sheets.
(b) | Included in prepayments and other current assets on the Consolidated Balance Sheets.
(c) | Included in deferred credits and other liabilities - other on the Consolidated Balance Sheets.
(d) | Included in other accrued liabilities on the Consolidated Balance Sheets.
(e) | Recovered as expense is incurred or cash contributions are made.
Regulatory liabilities | The following table summarizes the individual components of unamortized regulatory assets and liabilities as of December 31:
| Estimated Recovery Period | * | 2017 |  | 2016 |
|  |  | (In thousands)
Regulatory assets: |  |  |  |
Pension and postretirement benefits (a) | (e) |  | $ | 163,896 |  | $ | 176,025 |
Taxes recoverable from customers (a) | Over plant lives |  | 12,073 |  | 28,278 |
Manufactured gas plant sites remediation (a) | - |  | 18,213 |  | 18,259 |
Asset retirement obligations (a) | Over plant lives |  | 56,078 |  | 42,580 |
Natural gas costs recoverable through rate adjustments (b) | Up to 1 year |  | 14,465 |  | 2,242 |
Long-term debt refinancing costs (a) | Up to 20 years |  | 5,563 |  | 6,248 |
Costs related to identifying generation development (a) | Up to 9 years |  | 2,960 |  | 3,407 |
Other (a) (b) | Up to 20 years |  | 27,715 |  | 30,281 |
Total regulatory assets |  |  | 300,963 |  | 307,320 |
Regulatory liabilities: |  |  |  |
Plant removal and decommissioning costs (c) |  |  | 176,190 |  | 176,972 |
Taxes refundable to customers (c) |  |  | 279,668 |  | 11,010 |
Pension and postretirement benefits (c) |  |  | 11,056 |  | 9,099 |
Natural gas costs refundable through rate adjustments (d) |  |  | 28,514 |  | 25,580 |
Other (c) (d) |  |  | 23,870 |  | 19,191 |
Total regulatory liabilities |  |  | 519,298 |  | 241,852 |
Net regulatory position |  |  | $ | (218,335 | ) | $ | 65,468 |
* | Estimated recovery period for regulatory assets currently being recovered in rates charged to customers.
(a) | Included in deferred charges and other assets - other on the Consolidated Balance Sheets.
(b) | Included in prepayments and other current assets on the Consolidated Balance Sheets.
(c) | Included in deferred credits and other liabilities - other on the Consolidated Balance Sheets.
(d) | Included in other accrued liabilities on the Consolidated Balance Sheets.
(e) | Recovered as expense is incurred or cash contributions are made.

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- Definition Tabular disclosure of assets that are created when regulatory agencies permit public utilities to defer certain costs included in rate-setting to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44264-110382 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 715 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501251&loc=d3e52485-110419 Reference 3: 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 Tabular disclosure of regulatory liabilities. Detailed information about liabilities that result from rate actions of a regulator. Rate actions of a regulator can impose a liability on a regulated enterprise resulting in a regulatory liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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