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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
Regulatory Assets	$ 1,115,098	$ 845,471
Regulatory liabilities, natural gas derivative contracts	580,630	601,518
Swap
Derivative [Line Items]
Regulatory Assets	911	228
Regulatory liabilities, natural gas derivative contracts	0	271
Options
Derivative [Line Items]
Regulatory Assets	68	50
Regulatory liabilities, natural gas derivative contracts	0	76
Forward contracts
Derivative [Line Items]
Regulatory Assets	0	693
Regulatory liabilities, natural gas derivative contracts	$ 197	$ 0

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+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
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-Topic 980
-SubTopic 405
-Section 25
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