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Significant accounting policies (Tables)
12 Months Ended
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