Rate Matters and Regulation Rate Matters and Regulation (Details) - USD ($) $ in Millions		3 Months Ended	6 Months Ended	12 Months Ended
		Mar. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Public Utilities, Approved Equity Capital Structure, Percentage			53.00%
Public Utilities, Approved Debt Capital Structure, Percentage			47.00%
Amount of Interim Rate Increase Refunded			$ 15.3
Interim Rate Refund Amount			47.5
Current Regulatory Assets			171.0	$ 121.8
Dry Scrubber Project [Member]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress			323.4
Estimated Environmental Capital Costs			542.4
Mustang Modernization [Member]
Public Utilities, Property, Plant and Equipment, Construction Work in Progress			276.3
Estimated Environmental Capital Costs			390.0
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
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 8.8
Public Utilities, Approved Return on Equity, Percentage		9.50%
Approved Depreciation Reduction from ALJ Recommendation		$ 28.6
ALJ Depreciation Recommendation		$ 36.4
Approved Recovery of Short-term Incentive Comp		50.00%
Approved Recovery of Long-term Incentive Comp		0.00%
Interim Rate Revenue Reserved			5.6
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
Settlement of Rate Increase		$ 7.1
Settlement of Return on Equity, Percentage		9.50%
Settlement of Equity Capital Structure, Percentage		50.00%
Administrative Law Judge [Member]
Recommended Return on Equity				9.87%
Recommended Capital Structure, Equity Percentage				53.00%
Recommended Capital Structure, Debt Percentage				47.00%
Recommended Increase (Decrease) in Revenue				$ 40.7
Oklahoma demand program rider under recovery (A)
Current Regulatory Assets	[1]		$ 45.1	$ 51.0

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