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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Regulatory assets, natural gas derivative contracts	$ 450,474	$ 441,526
Regulatory liabilities, natural gas derivative contracts	578,592	576,124
Swap
Derivative [Line Items]
Regulatory assets, natural gas derivative contracts	66	0
Regulatory liabilities, natural gas derivative contracts	218	287
Options
Derivative [Line Items]
Regulatory liabilities, natural gas derivative contracts	134	138
Forward contracts
Derivative [Line Items]
Regulatory assets, natural gas derivative contracts	0	6,319
Regulatory liabilities, natural gas derivative contracts	$ 1,259	$ 0

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- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: 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

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ 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

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