SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)	12 Months Ended
	Dec. 31, 2014 item segment
Summary of Significant Accounting Policies
Number of business segments	3
Number of towns to which water service is provided	3
Electric
Operating revenues
% of revenue (as a percent)	90.80%
Number of customers	170,000
Electric | Water
Operating revenues
% of revenue (as a percent)	0.30%
Electric | Missouri
Operating revenues
% of revenue (as a percent)	89.70%
Electric | Kansas
Operating revenues
% of revenue (as a percent)	4.80%
Electric | Arkansas
Operating revenues
% of revenue (as a percent)	2.70%
Electric | Oklahoma
Operating revenues
% of revenue (as a percent)	2.80%
Electric | Residential
Operating revenues
% of revenue (as a percent)	40.00%
Electric | Commercial
Operating revenues
% of revenue (as a percent)	29.20%
Electric | Industrial
Operating revenues
% of revenue (as a percent)	14.40%
Electric | Wholesale on-system
Operating revenues
% of revenue (as a percent)	3.80%
Electric | Wholesale off-system
Operating revenues
% of revenue (as a percent)	7.60%
Electric | Miscellaneous sources, primarily public authorities
Operating revenues
% of revenue (as a percent)	2.60%
Electric | Other electric revenues
Operating revenues
% of revenue (as a percent)	2.40%
Gas
Operating revenues
% of revenue (as a percent)	8.00%
Number of customers	43,500
Gas | Residential
Operating revenues
% of revenue (as a percent)	63.40%
Gas | Commercial
Operating revenues
% of revenue (as a percent)	26.30%
Gas | Industrial
Operating revenues
% of revenue (as a percent)	1.00%
Gas | Other electric revenues
Operating revenues
% of revenue (as a percent)	9.30%
Others
Operating revenues
% of revenue (as a percent)	1.20%