Rate Matters and Regulation (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Public Utilities, Approved Equity Capital Structure, Percentage	53.00%
Public Utilities, Approved Debt Capital Structure, Percentage	47.00%
Current Regulatory Assets	$ 84.2	$ 121.8
Amount of Interim Rate Increase Refunded	45.6
Interim Rate Refund Amount	47.5
Dry Scrubber Project [Member]
Estimated Environmental Capital Costs	542.4
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	352.7
Mustang Modernization [Member]
Estimated Environmental Capital Costs	390.0
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	306.0
OKLAHOMA
Approved Depreciation Reduction from ALJ Recommendation	28.6
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 92.5
Public Utilities, Requested Return on Equity, Percentage	10.25%
Public Utilities, Requested Equity Capital Structure, Percentage	53.00%
Investments Since Last Rate Case	$ 1,600.0
Public Utilities, Interim Rate Increase (Decrease), Amount	69.5
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 8.8
Public Utilities, Approved Return on Equity, Percentage	9.50%
ALJ Depreciation Recommendation	$ 36.4
Approved Recovery of Short-term Incentive Comp	50.00%
Approved Recovery of Long-term Incentive Comp	0.00%
Interim Rate Revenue Reserved	$ 5.6
ARKANSAS
Settlement of Return on Equity, Percentage	9.50%
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 16.5
Public Utilities, Requested Return on Equity, Percentage	10.25%
Investments Since Last Rate Case	$ 3,000.0
Settlement of Rate Increase	$ 7.1
Settlement of Equity Capital Structure, Percentage	50.00%
Administrative Law Judge [Member]
Recommended Return on Equity	9.87%
Recommended Increase (Decrease) in Revenue	$ 40.7
Recommended Capital Structure, Equity Percentage	53.00%
Recommended Capital Structure, Debt Percentage	47.00%