Rate Matters and Regulation (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Estimated Refund to SPP	$ 13.0
Public Utilities, Amount Requested for Acquisition	53.5
Dry Scrubber Project [Member]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	512.4
Muskogee Natural Gas Conversion [Member]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	57.0
OKLAHOMA
Public Utilities, Requested Rate Increase (Decrease), Amount	77.6
ARKANSAS
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 3.3
FERC [Member]
Recommended Common Equity Percentage	7.85%
Public Utilities, Approved Return on Equity, Percentage	10.60%
Impact to Company [Member]
Estimated Refund to SPP	$ 5.0
Customer Impact [Member]
Estimated Refund to SPP	$ 8.0