Rate Matters and Regulation (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Estimated Refund to SPP	$ 21.0
Public Utilities, Amount Requested for Acquisition	53.5
Dry Scrubber Project [Member]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	511.3
Muskogee Natural Gas Conversion [Member]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	56.2
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	$ 5.5
Customer Impact [Member]
Estimated Refund to SPP	$ 15.5