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Property, Plant And Equipment (Major Classifications Of Property, Plant And Equipment) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]    
Construction work-in-progress (CWIP) and other $ 157,839 $ 150,474
Total 6,011,147 5,656,973
Other intangibles, property and investments-net 83,912 72,224
Total 6,047,930 5,687,737
Avista Utilities [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Total 5,783,918 5,434,977
Avista Utilities [Member] | Electric [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Production 1,392,017 1,346,332
Transmission 726,240 682,529
Distribution 1,617,451 1,525,175
Construction work-in-progress (CWIP) and other 322,144 296,912
Total 4,057,852 3,850,948
Avista Utilities [Member] | Natural Gas [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Distribution 1,027,197 954,298
Construction work-in-progress (CWIP) and other 63,803 57,601
Natural gas underground storage 46,233 44,672
Total 1,137,233 1,056,571
Avista Utilities [Member] | Common Plant [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Total 588,833 527,458
Alaska Electric Light & Power [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Total 227,229 221,996
Alaska Electric Light & Power [Member] | Electric [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Production 97,883 94,839
Transmission 21,413 20,252
Distribution 21,061 20,057
Capital Lease Assets 71,007 71,007
Construction work-in-progress (CWIP) and other 7,341 7,190
Total 218,705 213,345
Alaska Electric Light & Power [Member] | Common Plant [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Total 8,524 8,651
Corporate and Other [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Other intangibles, property and investments-net 36,783 30,764
Accumulated depreciation $ 11,600 $ 11,200