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ORGANIZATION AND BASIS OF PRESENTATION - Fuel Inventory and Supplies (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Inventory [Line Items]    
Fuel inventory $ 132.2 $ 168.9
Supplies 334.3 342.1
Fuel inventory and supplies 466.5 511.0
Westar Energy    
Inventory [Line Items]    
Fuel inventory 74.2 87.8
Supplies 180.8 189.0
Fuel inventory and supplies 255.0 276.8
KCP&L    
Inventory [Line Items]    
Fuel inventory 39.2 57.8
Supplies 120.1 119.8
Fuel inventory and supplies $ 159.3 $ 177.6