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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) |  |

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