Jointly Owned Electric Utility Plants (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Nuclear fuel, net	$ 76.6	$ 79.2
Kansas City Power and Light Company [Member]
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Nuclear fuel, net	76.6	79.2
Jointly Owned Electricity Generation Plant [Member] | Wolf Creek Unit [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	47.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	1,473.8
Accumulated depreciation	776.3
Nuclear fuel, net	76.6
Construction work in progress	39.4
Accredited capacity (in MegaWatts)	547
Jointly Owned Electricity Generation Plant [Member] | Wolf Creek Unit [Member] | Kansas City Power and Light Company [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	47.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	1,473.8
Accumulated depreciation	776.3
Nuclear fuel, net	76.6
Construction work in progress	39.4
Accredited capacity (in MegaWatts)	547
Jointly Owned Electricity Generation Plant [Member] | LaCygne Units [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	50.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	493.6
Accumulated depreciation	303.1
Construction work in progress	79.1
Accredited capacity (in MegaWatts)	711
Jointly Owned Electricity Generation Plant [Member] | LaCygne Units [Member] | Kansas City Power and Light Company [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	50.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	493.6
Accumulated depreciation	303.1
Construction work in progress	79.1
Accredited capacity (in MegaWatts)	711
Jointly Owned Electricity Generation Plant [Member] | Iatan No. 1 [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	88.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	667.9
Accumulated depreciation	252.8
Construction work in progress	6.5
Accredited capacity (in MegaWatts)	620
Jointly Owned Electricity Generation Plant [Member] | Iatan No. 1 [Member] | Kansas City Power and Light Company [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	70.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	542.3
Accumulated depreciation	207.9
Construction work in progress	2.6
Accredited capacity (in MegaWatts)	493
Jointly Owned Electricity Generation Plant [Member] | Iatan No. 2 Unit [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	73.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	1,293.0
Accumulated depreciation	270.0
Construction work in progress	5.9
Accredited capacity (in MegaWatts)	641
Jointly Owned Electricity Generation Plant [Member] | Iatan No. 2 Unit [Member] | Kansas City Power and Light Company [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	55.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	985.1
Accumulated depreciation	261.3
Construction work in progress	4.4
Accredited capacity (in MegaWatts)	482
Jointly Owned Electricity Generation Plant [Member] | Iatan Common [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	79.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	364.4
Accumulated depreciation	30.4
Construction work in progress	30.2
Jointly Owned Electricity Generation Plant [Member] | Iatan Common [Member] | Kansas City Power and Light Company [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	61.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	287.5
Accumulated depreciation	26.0
Construction work in progress	9.3
Jointly Owned Electricity Generation Plant [Member] | Jeffrey Energy Center [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership share (in hundredths)	8.00%
Jointly owned utility plant, gross ownership amount of plant in service [Abstract]
Utility plant in service	158.4
Accumulated depreciation	75.5
Construction work in progress	$ 5.3
Accredited capacity (in MegaWatts)	174