Rate Matters and Regulation Rate Matters and Regulation (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2013
Annual Recovery of Lost Revenues Through Rate Implementation	$ 10.0
Additional Requested Plant Dismantlement Cost	$ 8.0
Change in Requested Return on Equity	0.25%
Change in Requested Rate Increase	$ 9.0
SmartHours Program [Member]
Lost Revenue Associated with Customer Programs	36.6	$ 30.3	$ 10.1
Dry Scrubber Project [Member]
Estimated Environmental Capital Costs	547.5
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	138.6
Mustang Modernization [Member]
Estimated Environmental Capital Costs	424.9
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	133.6
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
Interim Rate Collected	23.6
Interim Rate Revenue Reserved	21.0
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
Oklahoma Attorney General [Member]
Reduction to Requested Additional Plant Dismantlement Costs	$ 20.9
Oklahoma Public Utility Division Staff [Member]
Recommended Return on Equity	9.25%
Recommended Common Equity Percentage	53.00%
Reduction to Requested Additional Plant Dismantlement Costs	$ 33.0
Recommended Rate Increase (Decrease)	$ 6.1
Oklahoma Industrial Electric Consumers [Member]
Recommended Return on Equity	9.00%
Recommended Common Equity Percentage	53.00%
Reduction to Requested Additional Plant Dismantlement Costs	$ 52.5
Recommended Rate Increase (Decrease)	$ 47.9
Recommendation 1 [Member] | Oklahoma Attorney General [Member]
Recommended Return on Equity	9.25%
Recommended Common Equity Percentage	50.00%
Recommended Rate Increase (Decrease)	$ 10.8
Recommendation 2 [Member] | Oklahoma Attorney General [Member]
Recommended Return on Equity	8.90%
Recommended Common Equity Percentage	53.00%
Recommended Rate Increase (Decrease)	$ 13.7
Change in Allowed Amount [Member] | SmartHours Program [Member]
Lost Revenue Associated with Customer Programs	$ (6.3)