Rate Matters and Regulation Rate Matters and Regulation (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 64.0
Public Utilities, Approved Return on Equity, Percentage		9.50%
Public Utilities, Approved Debt Capital Structure, Percentage		47.00%
Public Utilities, Approved Equity Capital Structure, Percentage		53.00%
Customer Refund Liability, Current		$ 39.2	$ 1.7
Interim Rate Revenue Reserved		22.8
Current Regulatory Assets		18.4	40.8
Recommended Disallowance for Fuel Adjustment		3.3
Refund for Fuel Adjustment		2.4
Public Utilities, Requested Rate Increase (Decrease), Amount		6.4
Dry Scrubber Project [Member]
Estimated Environmental Capital Costs		532.0
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		483.5
OKLAHOMA
Customer Refund Liability, Current		18.9
Interim Rate Revenue Reserved		5.6
ARKANSAS
Customer Refund Liability, Current		7.7
Oklahoma demand program rider under recovery (A)
Current Regulatory Assets	[1]	$ 10.7	$ 31.6
January 16, 2018 [Member] | OKLAHOMA
Public Utilities, Requested Return on Equity, Percentage		9.90%
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 1.9
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.