Rate Matters and Regulation (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 6.4
Public Utilities, Approved Rate Increase (Decrease), Amount		$ (64.0)
Public Utilities, Approved Return on Equity, Percentage		9.50%
Public Utilities, Approved Equity Capital Structure, Percentage		53.00%
Public Utilities, Approved Debt Capital Structure, Percentage		47.00%
Current Regulatory Assets		$ 18.4	$ 40.8
Interim Rate Revenue Reserved		22.8
Recommended Disallowance for Fuel Adjustment		3.3
Refund for Fuel Adjustment		2.4
Customer Refund Liability, Current		39.2	1.7
Dry Scrubber Project [Member]
Estimated Environmental Capital Costs		532.0
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		483.5
OKLAHOMA
Interim Rate Revenue Reserved		5.6
Customer Refund Liability, Current		18.9
ARKANSAS
Customer Refund Liability, Current		7.7
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]
Recommended Common Equity Percentage		7.85%
Public Utilities, Approved Return on Equity, Percentage		10.60%
Revenue impact of recommended change in return on common equity		$ 1.5
Oklahoma demand program rider under recovery [Member]
Current Regulatory Assets	[1]	$ 10.7	$ 31.6

[1] Included in Other Current Assets on the Condensed Balance Sheets.