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Significant accounting policies - Capitalization of Interest (Detail) - USD ($) $ in Thousands | 12 Months Ended
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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

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- 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 + Details
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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 + Details
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- Definition Amount capitalized 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 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430 + Details
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