Rate Matters and Regulation (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 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 Equity Capital Structure, Percentage	53.00%
Public Utilities, Approved Debt Capital Structure, Percentage	47.00%
Current Regulatory Assets	$ 19.7	$ 40.8
Interim Rate Revenue Reserved	32.8
Recommended Disallowance for Fuel Adjustment	3.3
Refund for Fuel Adjustment	2.4
Dry Scrubber Project [Member]
Estimated Environmental Capital Costs	542.4
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	451.6
Mustang Modernization [Member]
Estimated Environmental Capital Costs	385.0
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	376.3
OKLAHOMA
Interim Rate Revenue Reserved	5.6
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]
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