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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 |  |  |

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