XML 64 R54.htm IDEA: XBRL DOCUMENT v3.8.0.1
Rate Matters and Regulation Rate Matters and Regulation (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Interim Rate Revenue Reserved $ 6.5  
Current Regulatory Assets 31.6 $ 40.8
Recommended Disallowance for Fuel Adjustment 3.3  
Refund for Fuel Adjustment 2.4  
Dry Scrubber Project [Member]    
Public Utilities, Property, Plant and Equipment, Construction Work in Progress 416.0  
Estimated Environmental Capital Costs 542.4  
Mustang Modernization [Member]    
Public Utilities, Property, Plant and Equipment, Construction Work in Progress 373.2  
Estimated Environmental Capital Costs 390.0  
OKLAHOMA    
Interim Rate Revenue Reserved 5.6  
Oklahoma demand program rider under recovery (A)    
Current Regulatory Assets [1] 23.9 $ 31.6
January 16, 2018 [Member] | OKLAHOMA    
Public Utilities, Requested Rate Increase (Decrease), Amount $ 1.9  
Public Utilities, Requested Return on Equity, Percentage 9.90%  
FERC [Member]    
Public Utilities, Approved Return on Equity, Percentage 10.60%  
Recommended Common Equity Percentage 7.85%  
Revenue impact of recommended change in return on common equity $ 1.5  
[1] Included in Other Current Assets on the Condensed Consolidated Balance Sheets.