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Rate Matters and Regulation Rate Matters and Regulation (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Interim Rate Revenue Reserved |  | $ 6.5 |
Current Regulatory Assets |  | 31.6 | $ 40.8
Recommended Disallowance for Fuel Adjustment |  | 3.3 |
Refund for Fuel Adjustment |  | 2.4 |
Dry Scrubber Project [Member] |  |  |
Public Utilities, Property, Plant and Equipment, Construction Work in Progress |  | 416.0 |
Estimated Environmental Capital Costs |  | 542.4 |
Mustang Modernization [Member] |  |  |
Public Utilities, Property, Plant and Equipment, Construction Work in Progress |  | 373.2 |
Estimated Environmental Capital Costs |  | 390.0 |
OKLAHOMA |  |  |
Interim Rate Revenue Reserved |  | 5.6 |
Oklahoma demand program rider under recovery (A) |  |  |
Current Regulatory Assets | [1] | 23.9 | $ 31.6
January 16, 2018 [Member] | OKLAHOMA |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  | $ 1.9 |
Public Utilities, Requested Return on Equity, Percentage |  | 9.90% |
FERC [Member] |  |  |
Public Utilities, Approved Return on Equity, Percentage |  | 10.60% |
Recommended Common Equity Percentage |  | 7.85% |
Revenue impact of recommended change in return on common equity |  | $ 1.5 |
[1] | Included in Other Current Assets on the Condensed Consolidated Balance Sheets.

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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