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Significant accounting policies (Tables) | 12 Months Ended
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Dec. 31, 2017
Accounting Policies [Abstract] |
Capitalization of Interest | The AFUDC capitalized that relates to equity funds is recorded as interest, dividend, equity and other income on the consolidated statements of operations.
| 2017 |  | 2016
Interest capitalized on non-regulated property | $ | 5,558 |  |  | $ | 3,259 |
AFUDC capitalized on regulated property: |  |  |
Allowance for borrowed funds | 1,673 |  |  | 1,167 |
Allowance for equity funds | 3,014 |  |  | 2,774 |
Total | $ | 10,245 |  |  | $ | 7,200 |
Estimated Useful Lives of Depreciable Assets | The ranges of estimated useful lives and the weighted average useful lives are summarized below:
| Range of useful lives |  | Weighted average useful lives
| 2017 |  | 2016 |  | 2017 |  | 2016
Generation | 3 - 60 |  | 3 - 60 |  | 33 |  | 32
Distribution | 5 - 100 |  | 5 - 100 |  | 40 |  | 41
Equipment | 5 - 50 |  | 5 - 50 |  | 13 |  | 11

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- Definition Tabular disclosure of the capitalization of the entity comprised of its long-term debt and equity instruments. The table may be detailed by subsidiary (legal entity) and include information by type of debt or equity detailed by instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 270 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (a)(5) -URI http://asc.fasb.org/extlink&oid=27015980&loc=d3e46468-122699 + Details
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