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Regulatory assets and liabilities Regulatory Assets (Details 1) - USD ($) $ in Thousands | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2020 | Dec. 31, 2019 | Jun. 30, 2019
Regulatory Assets [Line Items] |  |  |
Regulatory Assets, Current | $ 60,557 | $ 63,613 | $ 80,661
Regulatory Assets, Noncurrent | 351,025 | 353,784 | 327,164
Total regulatory assets | 411,582 | 417,397 | 407,825
Regulatory assets not earning a rate of return | 296,600 | 276,500 | 290,700
Natural gas costs recoverable through rate adjustments |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets, Current | 42,582 | 42,823 | 65,448
Regulatory Assets, Noncurrent | 27,184 | 46,381 | 29,095
Cost recovery mechanisms |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets, Current | 7,781 | 6,288 | 7,484
Regulatory Assets, Noncurrent | $ 13,140 | 13,108 | 11,004
Conservation programs |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Up to 1 year |  |
Regulatory Assets, Current | $ 7,256 | 6,963 | 6,529
Pension and postretirement benefits |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | ** |  |
Regulatory Assets, Noncurrent | $ 157,033 | 157,069 | 165,861
Asset retirement obligations |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Over plant lives |  |
Regulatory Assets, Noncurrent | $ 68,191 | 66,000 | 64,215
Plant to be retired |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | - |  |
Regulatory Assets, Noncurrent | $ 49,185 | 32,931 | 16,933
Manufactured gas plant site remediation |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | - |  |
Regulatory Assets, Noncurrent | $ 14,826 | 15,126 | 15,387
Taxes recoverable from customers |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Over plant lives |  |
Regulatory Assets, Noncurrent | $ 10,890 | 11,486 | 11,755
Long-term debt refinancing costs |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Up to 17 years |  |
Regulatory Assets, Noncurrent | $ 3,980 | 4,286 | 4,592
Other Regulatory Assets |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets, Current | 2,938 | 7,539 | 1,200
Regulatory Assets, Noncurrent | $ 6,596 | $ 7,397 | $ 8,322
Minimum [Member] | Natural gas costs recoverable through rate adjustments |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Up to 1 year |  |
Minimum [Member] | Cost recovery mechanisms |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Up to 1 year |  |
Minimum [Member] | Other Regulatory Assets |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Up to 1 year |  |
Maximum [Member] | Natural gas costs recoverable through rate adjustments |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Up to 3 years |  |
Maximum [Member] | Cost recovery mechanisms |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Up to 10 years |  |
Maximum [Member] | Other Regulatory Assets |  |  |
Regulatory Assets [Line Items] |  |  |
Estimated Recovery Period as of June 30, 2020 | Up to 19 years |  |

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- Definition Describes the continual or periodic assessment of the probability of future recovery of regulatory assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 40 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499898&loc=d3e44174-110380 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121557228&loc=d3e43603-110378 + Details
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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=121594574&loc=d3e44360-110383 + Details
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