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Significant accounting policies - Capitalization of Interest (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule of Capitalization [Line Items]
Total	$ 5,846	$ 7,957
Non-regulated property
Schedule of Capitalization [Line Items]
Interest capitalized on non-regulated property	1,434	4,325
Interest expense
Schedule of Capitalization [Line Items]
AFUDC capitalized on regulated property	1,684	1,297
Interest, dividend and other income
Schedule of Capitalization [Line Items]
AFUDC capitalized on regulated property	$ 2,728	$ 2,335

X

- Definition

Amount of interest capitalized during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

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- Definition

Amount of interest capitalized, including amount of allowance for funds used during construction.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 835
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 835
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430

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