SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)	12 Months Ended
	Dec. 31, 2015 segment customer item
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Number of business segments | segment	3
Number of towns to which water service is provided | item	3
Electric
Operating revenues
% of revenue (as a percent)	91.70%
Number of customers	170,000
Electric | On-system Member
Operating revenues
% of revenue (as a percent)	86.60%
Electric | SPP IM
Operating revenues
% of revenue (as a percent)	2.50%
Electric | Other revenues
Operating revenues
% of revenue (as a percent)	2.30%
Electric | Water
Operating revenues
% of revenue (as a percent)	0.30%
Electric | Missouri
Operating revenues
% of revenue (as a percent)	89.00%
Electric | Kansas
Operating revenues
% of revenue (as a percent)	4.80%
Electric | Arkansas
Operating revenues
% of revenue (as a percent)	3.40%
Electric | Oklahoma
Operating revenues
% of revenue (as a percent)	2.80%
Electric | Residential
Operating revenues
% of revenue (as a percent)	41.70%
Electric | Commercial
Operating revenues
% of revenue (as a percent)	31.10%
Electric | Industrial
Operating revenues
% of revenue (as a percent)	15.90%
Electric | Wholesale on-system
Operating revenues
% of revenue (as a percent)	3.30%
Electric | Wholesale off-system
Operating revenues
% of revenue (as a percent)	2.70%
Electric | Miscellaneous sources, primarily public authorities
Operating revenues
% of revenue (as a percent)	2.80%
Electric | Miscellaneous
Operating revenues
% of revenue (as a percent)	2.50%
Gas
Operating revenues
% of revenue (as a percent)	6.90%
Number of customers	43,200
Gas | Residential
Operating revenues
% of revenue (as a percent)	63.00%
Gas | Commercial
Operating revenues
% of revenue (as a percent)	25.60%
Gas | Industrial
Operating revenues
% of revenue (as a percent)	0.80%
Gas | Transportation
Operating revenues
% of revenue (as a percent)	8.90%
Gas | Miscellaneous
Operating revenues
% of revenue (as a percent)	1.70%
Other
Operating revenues
% of revenue (as a percent)	1.40%