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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Regulatory assets, natural gas derivative contracts	$ 559,887	$ 460,095
Regulatory liabilities, natural gas derivative contracts	598,062	588,213
Swap
Derivative [Line Items]
Regulatory assets, natural gas derivative contracts	28	66
Regulatory liabilities, natural gas derivative contracts	743	218
Options
Derivative [Line Items]
Regulatory assets, natural gas derivative contracts	38	0
Regulatory liabilities, natural gas derivative contracts	0	134
Forward contracts
Derivative [Line Items]
Regulatory assets, natural gas derivative contracts	1,830	0
Regulatory liabilities, natural gas derivative contracts	$ 0	$ 1,259

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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
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