Rate Matters and Regulation Rate Matters and Regulation (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Dry Scrubber Project [Member]
Estimated Environmental Capital Costs		$ 547.5
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		281.6
Mustang Modernization [Member]
Estimated Environmental Capital Costs		424.9
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		231.8
OKLAHOMA
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%
Approved Depreciation Reduction from ALJ Recommendation	$ 28.6
ALJ Depreciation Recommendation	$ 36.4
Approved Recovery of Short-term Incentive Comp	50.00%
Approved Recovery of Long-term Incentive Comp	0.00%
ARKANSAS
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 Return on Equity, Percentage	9.50%
Settlement of Equity Capital Structure, Percentage	50.00%
Administrative Law Judge [Member]
Recommended Return on Equity		9.87%
Recommended Capital Structure, Equity Percentage		53.00%
Recommended Capital Structure, Debt Percentage		47.00%
Recommended Increase (Decrease) in Revenue		$ 40.7