Rate Matters and Regulation (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Annual Recovery of Lost Revenues Through Rate Implementation	$ 10.0
Additional Plant Dismantlement Costs	$ 8.0
Change in Requested Return on Equity	0.25%
Change in Requested Rate Increase	$ 9.0
Oklahoma Attorney General's Staff [Member]
Reduction to Requested Additional Plant Dismantlement Costs	20.9
Oklahoma Public Utility Division Staff [Member]
Recommended Rate Increase	$ 6.1
Recommended Return on Equity	9.25%
Recommended Common Equity Percentage	53.00%
Reduction to Requested Additional Plant Dismantlement Costs	$ 33.0
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 Decrease	47.9
Regional Haze [Member]
Estimated Environmental Capital Costs		$ 1,100.0
SmartHours Program [Member]
Lost Revenue Associated with Customer Programs	36.6	14.9	$ 11.6	$ 10.1
Remaining Lost Revenues to be Recovered Through Rates	24.0
Regulated Operating Revenue, Other	12.6
Oklahoma Public Utility Division Staff [Member] | SmartHours Program [Member]
Lost Revenue Associated with Customer Programs				2.6
Reduction of Revenue				$ 7.5
Environmental Compliance Plan [Member]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	107.3
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
Recommendation 1 [Member] | Oklahoma Attorney General's Staff [Member]
Recommended Return on Equity	9.25%
Recommended Common Equity Percentage	50.00%
Recommended Rate Decrease	$ 10.8
Recommendation 2 [Member] | Oklahoma Attorney General's Staff [Member]
Recommended Return on Equity	8.90%
Recommended Common Equity Percentage	53.00%
Recommended Rate Decrease	$ 13.7