COMMONLY OWNED FACILITIES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Commonly Owned Facilities
Proportionate ownership interest (as a percent)	12.00%
Distance of Iatan Generating Facility from Kansas city (in miles)	35
Ownership interest in the site (as a percent)	3.00%
EDE
Commonly Owned Facilities
Ownership interest in certain common facilities (as a percent)	12.00%
Iatan
Commonly Owned Facilities
Proportionate ownership interest (as a percent)	12.00%
Amounts included in property, plant and equipment
Cost of ownership in plant in service	$ 364.1	$ 362.6
Accumulated Depreciation	83.2	39.6
Expenditures	30.0	31.3
Iatan1 | KCP&L
Commonly Owned Facilities
Proportionate ownership interest (as a percent)	70.00%
Iatan1 | KCP&L Greater Missouri Operations Co.
Commonly Owned Facilities
Proportionate ownership interest (as a percent)	18.00%
Iatan2 | KCP&L
Commonly Owned Facilities
Proportionate ownership interest (as a percent)	54.00%
Iatan2 | KCP&L Greater Missouri Operations Co.
Commonly Owned Facilities
Proportionate ownership interest (as a percent)	18.00%
State Line Combined Cycle Unit
Amounts included in property, plant and equipment
Cost of ownership in plant in service	164.4	162.1
Accumulated Depreciation	36.7	32.1
Power capacity (in megawatts)	500
Expenditures	42.7	57.0
State Line Combined Cycle Unit | EDE
Commonly Owned Facilities
Percentage share in available capacity	60.00%
Amounts included in property, plant and equipment
Percentage share in operating costs	60.00%
Plum point
Amounts included in property, plant and equipment
Cost of ownership in plant in service	108.0	110.1
Accumulated Depreciation	4.9	2.7
Expenditures	$ 7.8	$ 8.5
Plum point | EDE
Commonly Owned Facilities
Proportionate ownership interest (as a percent)	7.52%