Regulatory Matters Asset (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	$ 1,120.9		$ 1,058.2
Regulatory Liability, Noncurrent	283.8		268.5
Taxes recoverable through future rates [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	138.4		144.2
Loss on reacquired debt [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	10.1		11.8
Cost of removal, asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	2.8		4.6
Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	46.4		45.2
Pension and post-retirement costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	670.9		588.4
Deferred customer programs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	74.4		68.8
Rate case expenses [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	9.2		13.4
Fuel recovery mechanisms [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	25.8		50.4
Acquisition transition costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	34.2		44.9
Derivative instruments [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	3.7		7.6
Iatan No. 1 and Common facilities depreciation and carrying costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	21.8		22.5
Iatan No. 2 construction accounting costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	46.8		43.3
Kansas property tax surcharge [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	5.4		3.7
Solar Rebates [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	15.8		0
Voluntary Separation Program [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	4.3		0
Other regulatory noncurrent assets [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	10.9		9.4
Kansas City Power and Light Company [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	853.2		780.7
Regulatory Liability, Noncurrent	153.0		142.8
Kansas City Power and Light Company [Member] | Taxes recoverable through future rates [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	114.7		119.6
Kansas City Power and Light Company [Member] | Loss on reacquired debt [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	8.1	[1]	9.1
Regulatory amortization period	over the life of the related new debt issuances or the remaining lives of the old debt issuances if no new debt was issued.
Kansas City Power and Light Company [Member] | Cost of removal, asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	2.8		4.6
Kansas City Power and Light Company [Member] | Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	31.5		31.4
Kansas City Power and Light Company [Member] | Pension and post-retirement costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	541.2	[2]	466.4
Amount not included in rate base	526.5
Regulatory amortization period	various
Kansas City Power and Light Company [Member] | Deferred customer programs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	49.8	[3]	48.2
Amount not included in rate base	5.0
Regulatory amortization period	various
Kansas City Power and Light Company [Member] | Rate case expenses [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	7.5	[4]	9.6
Regulatory amortization period	various
Kansas City Power and Light Company [Member] | Fuel recovery mechanisms [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	8.9	[4]	14.0
Regulatory amortization period	various
Kansas City Power and Light Company [Member] | Acquisition transition costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	18.7	[5]	24.7
Regulatory amortization period	through 2016
Kansas City Power and Light Company [Member] | Derivative instruments [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	0		0
Kansas City Power and Light Company [Member] | Iatan No. 1 and Common facilities depreciation and carrying costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	15.9	[6]	16.4
Regulatory amortization period	through 2038
Kansas City Power and Light Company [Member] | Iatan No. 2 construction accounting costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	30.6	[7]	27.9
Regulatory amortization period	through 2058
Kansas City Power and Light Company [Member] | Kansas property tax surcharge [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	5.4	[4]	3.7
Regulatory amortization period	various
Kansas City Power and Light Company [Member] | Solar Rebates [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	5.8	[8]	0
Regulatory amortization period	through 2015
Kansas City Power and Light Company [Member] | Voluntary Separation Program [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	4.3	[9]	0
Regulatory amortization period	through 2017
Kansas City Power and Light Company [Member] | Other regulatory noncurrent assets [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	8.0	[4]	5.1
Regulatory amortization period	various
KCPL Greater Missouri Operations [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	267.7		277.5
Regulatory Liability, Noncurrent	130.8		125.7
KCPL Greater Missouri Operations [Member] | Taxes recoverable through future rates [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	23.7		24.6
KCPL Greater Missouri Operations [Member] | Loss on reacquired debt [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	2.0	[1]	2.7
Regulatory amortization period	over the life of the related new debt issuances or the remaining lives of the old debt issuances if no new debt was issued.
KCPL Greater Missouri Operations [Member] | Cost of removal, asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	0		0
KCPL Greater Missouri Operations [Member] | Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	14.9		13.8
KCPL Greater Missouri Operations [Member] | Pension and post-retirement costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	129.7	[2]	122.0
Amount not included in rate base	127.1
Regulatory amortization period	various
KCPL Greater Missouri Operations [Member] | Deferred customer programs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	24.6	[10]	20.6
Regulatory amortization period	through 2019
KCPL Greater Missouri Operations [Member] | Rate case expenses [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	1.7	[11]	3.8
Regulatory amortization period	through 2014
KCPL Greater Missouri Operations [Member] | Fuel recovery mechanisms [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	16.9	[4]	36.4
Regulatory amortization period	various
KCPL Greater Missouri Operations [Member] | Acquisition transition costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	15.5	[5]	20.2
Regulatory amortization period	2016
KCPL Greater Missouri Operations [Member] | Derivative instruments [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	3.7	[12]	7.6
KCPL Greater Missouri Operations [Member] | Iatan No. 1 and Common facilities depreciation and carrying costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	5.9	[6]	6.1
Regulatory amortization period	through 2038
KCPL Greater Missouri Operations [Member] | Iatan No. 2 construction accounting costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	16.2	[7]	15.4
Regulatory amortization period	through 2058
KCPL Greater Missouri Operations [Member] | Kansas property tax surcharge [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	0		0
KCPL Greater Missouri Operations [Member] | Solar Rebates [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	10.0	[8]	0
Regulatory amortization period	through 2015
KCPL Greater Missouri Operations [Member] | Voluntary Separation Program [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	0		0
KCPL Greater Missouri Operations [Member] | Other regulatory noncurrent assets [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	$ 2.9	[4]	$ 4.3
Regulatory amortization period	various

[1] (a) Amortized over the life of the related new debt issuances or the remaining lives of the old debt issuances if no new debt was issued.

[2] (b) Represents unrecognized gains and losses, prior service and transition costs that will be recognized in future net periodic pension and post-retirement costs, pension settlements amortized over various periods and financial and regulatory accounting method differences that will be eliminated over the life of the pension plans. Of these amounts, $526.5 million and $127.1 million for KCP&L and GMO, respectively, are not included in rate base and are amortized over various periods.

[3] (c) $5.0 million not included in rate base and amortized over various periods.

[4] (e) Not included in rate base and amortized over various periods.

[5] (g) Not included in rate base and amortized through 2016.

[6] (i) Included in rate base and amortized through 2038.

[7] (j) Included in rate base and amortized through 2058.

[8] (k) Not included in rate base and amortized through 2015.

[9] (l) Not included in rate base and amortized through 2017.

[10] (d) Included in rate base and amortized through 2019.

[11] (f) Not included in rate base and amortized through 2014.

[12] (h) Represents the fair value of derivative instruments for commodity contracts. Settlements of the contracts are recognized in fuel expense and included in GMO's Fuel Adjustment Clause (FAC).