Rate Matters and Regulation (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended	12 Months Ended
	Jul. 31, 2016	Jun. 30, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Contributions	$ 20.0
Annual Recovery of Lost Revenues Through Rate Implementation		$ 10.0
Additional Requested Plant Dismantlement Cost		$ 8.0
Change in Requested Return on Equity		0.25%
Change in Requested Rate Increase		$ 9.0
Public Utilities, Interim Rate Increase (Decrease), Amount		69.5
SmartHours Program [Member]
Lost Revenue Associated with Customer Programs		36.6	$ 14.9	$ 11.6	$ 10.1
Remaining Lost Revenues to be Recovered Through Rates		21.4
Lost Revenue Collected from Customers		15.2
Oklahoma Public Utility Division Staff [Member] | SmartHours Program [Member]
Lost Revenue Associated with Customer Programs					2.6
Reduction of Revenue					$ 7.5
Dry Scrubber Project [Member]
Estimated Environmental Capital Costs		547.5
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		116.8
Mustang Modernization [Member]
Estimated Environmental Capital Costs		424.9
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		103.6
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
Oklahoma Attorney General [Member]
Reduction to Requested Additional Plant Dismantlement Costs		$ 20.9
Oklahoma Public Utility Division Staff [Member]
Recommended Rate Increase (Decrease)		$ 6.1
Recommended Return on Equity		9.25%
Recommended Common Equity Percentage		53.00%
Reduction to Requested Additional Plant Dismantlement Costs		$ 33.0
Oklahoma Industrial Electric Consumers [Member]
Recommended Rate Increase (Decrease)		$ 47.9
Recommended Return on Equity		9.00%
Recommended Common Equity Percentage		53.00%
Reduction to Requested Additional Plant Dismantlement Costs		$ 52.5
Recommendation 1 [Member] | Oklahoma Attorney General [Member]
Recommended Rate Increase (Decrease)		$ 10.8
Recommended Return on Equity		9.25%
Recommended Common Equity Percentage		50.00%
Recommendation 2 [Member] | Oklahoma Attorney General [Member]
Recommended Rate Increase (Decrease)		$ 13.7
Recommended Return on Equity		8.90%
Recommended Common Equity Percentage		53.00%