REGULATORY MATTERS (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Regulatory Assets [Line Items]
Non current assets amount	$ 977.2		$ 924.0
Regulatory Liabilities [Line Items]
Non current liabilities	257.8		258.2
Kansas City Power and Light Company [Member] | Rate jurisdiction - Kansas [Member]
Regulatory Proceedings [Line Items]
Return on equity	10.00%
Rate-making equity ratio	49.70%
Effective date	Dec. 01, 2010
Annual revenue increase authorized	21.8
Jurisdictional rate base	1,781
Adjusted annual revenue increased authorized	22.0
Annual revenue increase authorized but considered as interim subject to refund or true-up	1.4
Kansas City Power and Light Company [Member] | Rate jurisdiction - Missouri [Member]
Regulatory Proceedings [Line Items]
File date	Jun. 04, 2010
Annual revenue increase	92.1
Adjusted annual revenue increase	66.5
Return on equity	10.00%
Rate-making equity ratio	46.30%
Effective date	May 04, 2011
Annual revenue increase authorized	34.8
Jurisdictional rate base	2,000
Wholesale margin offset requested	29.4
Wholesale margin offset authorized	45.9
Disallowed construction costs on Iatan Nos 1 and 2	1.5
Other disallowed costs	2.4
Qualifying advance coal project tax credits to be reallocated from KCPL to GMO	26.5
KCPL Greater Missouri Operations Company [Member] | Rate jurisdiction - Missouri [Member]
Regulatory Proceedings [Line Items]
Jurisdictional rate base	1,760
Disallowed construction costs on Iatan Nos 1 and 2	0.8
Other disallowed costs	1.5
KCPL Greater Missouri Operations Company [Member] | Rate jurisdiction - Missouri public service division [Member]
Regulatory Proceedings [Line Items]
File date	Jun. 04, 2010
Annual revenue increase	75.8
Adjusted annual revenue increase	65.9
Return on equity	10.00%
Rate-making equity ratio	46.60%
Effective date	Jun. 25, 2011
Annual revenue increase authorized	30.1
Adjusted annual revenue increased authorized	35.7
Amount of Crossroads disallowed from rate base	50.0
Amount of Crossroads transmission expense disallowed	4.9
Amount of Crossroads accumulated deferred taxes offset against rate base	15.0
Amount of Crossroads requested in rate base	104.0
KCPL Greater Missouri Operations Company [Member] | Rate jurisdiction - St. Joseph Light and Power division [Member]
Regulatory Proceedings [Line Items]
File date	Jun. 04, 2010
Annual revenue increase	22.1
Adjusted annual revenue increase	23.2
Return on equity	10.00%
Rate-making equity ratio	46.60%
Effective date	Jun. 25, 2011
Annual revenue increase authorized	29.3
Adjusted annual revenue increased authorized	29.8
Deferred annual revenue increase	7.7
Kansas City Power and Light Company [Member]
Regulatory Proceedings [Line Items]
Deferred tax credits (note)	128.4
Regulatory Assets [Line Items]
Non current assets amount	700.6		679.6
Regulatory Liabilities [Line Items]
Non current liabilities	135.6		141.3
Kansas City Power and Light Company [Member] | Taxes recoverable through future rates [Member]
Regulatory Assets [Line Items]
Non current assets amount	119.0		117.2
Kansas City Power and Light Company [Member] | Loss on reacquired debt [Member]
Regulatory Assets [Line Items]
Non current assets amount	6.5	[1]	5.0
Kansas City Power and Light Company [Member] | Cost of removal, asset [Member]
Regulatory Assets [Line Items]
Non current assets amount	7.0		8.5
Kansas City Power and Light Company [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Non current assets amount	30.4		27.5
Kansas City Power and Light Company [Member] | Pension and post-retirement costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	373.2	[2]	386.1
Kansas City Power and Light Company [Member] | Deferred customer programs [Member]
Regulatory Assets [Line Items]
Non current assets amount	48.2	[3]	44.7
Amount not included in rate base	11.3
Regulatory amortization period	various
Kansas City Power and Light Company [Member] | Rate case expenses [Member]
Regulatory Assets [Line Items]
Non current assets amount	9.9	[4]	12.3
Regulatory amortization period	various
Kansas City Power and Light Company [Member] | Skill set realignment costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	3.7	[5]	4.8
Amount not included in rate base	2.5
Regulatory amortization period	2017
Kansas City Power and Light Company [Member] | Fuel adjustment clauses [Member]
Regulatory Assets [Line Items]
Non current assets amount	30.1	[4]	8.4
Kansas City Power and Light Company [Member] | Acquisition transition costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	26.2	[6]	29.3
Regulatory amortization period	2016
Kansas City Power and Light Company [Member] | St. Joseph Light and Power acquisition [Member]
Regulatory Assets [Line Items]
Non current assets amount	0		0
Kansas City Power and Light Company [Member] | Storm damage [Member]
Regulatory Assets [Line Items]
Non current assets amount	0		0
Kansas City Power and Light Company [Member] | Derivative instruments [Member]
Regulatory Assets [Line Items]
Non current assets amount	0		0
Kansas City Power and Light Company [Member] | Iatan No. 1 and Common facilities depreciation and carrying costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	16.5		15.1
Kansas City Power and Light Company [Member] | Iatan No. 2 construction accounting costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	27.6		17.2
Kansas City Power and Light Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Non current assets amount	2.3	[7]	3.5
Regulatory amortization period	various
Kansas City Power and Light Company [Member] | Emission allowances [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	83.0		85.9
Kansas City Power and Light Company [Member] | Asset retirement obligations [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	40.9		44.9
Kansas City Power and Light Company [Member] | Pension [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	0.5		0
Kansas City Power and Light Company [Member] | Cost of removal, liability [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	0		0
Kansas City Power and Light Company [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	11.2		10.5
KCPL Greater Missouri Operations Company [Member]
Regulatory Proceedings [Line Items]
Deferred tax credits (note)	3.5
Regulatory Assets [Line Items]
Non current assets amount	276.6		244.4
Regulatory Liabilities [Line Items]
Non current liabilities	122.2		116.9
KCPL Greater Missouri Operations Company [Member] | Taxes recoverable through future rates [Member]
Regulatory Assets [Line Items]
Non current assets amount	24.9		25.3
KCPL Greater Missouri Operations Company [Member] | Loss on reacquired debt [Member]
Regulatory Assets [Line Items]
Non current assets amount	0.6	[1]	0.7
KCPL Greater Missouri Operations Company [Member] | Cost of removal, asset [Member]
Regulatory Assets [Line Items]
Non current assets amount	0		0
KCPL Greater Missouri Operations Company [Member] | Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Non current assets amount	13.6		12.8
KCPL Greater Missouri Operations Company [Member] | Pension and post-retirement costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	120.4	[2]	106.7
KCPL Greater Missouri Operations Company [Member] | Deferred customer programs [Member]
Regulatory Assets [Line Items]
Non current assets amount	19.5		15.6
KCPL Greater Missouri Operations Company [Member] | Rate case expenses [Member]
Regulatory Assets [Line Items]
Non current assets amount	4.5	[4]	3.3
Regulatory amortization period	various
KCPL Greater Missouri Operations Company [Member] | Skill set realignment costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	0		0
KCPL Greater Missouri Operations Company [Member] | Fuel adjustment clauses [Member]
Regulatory Assets [Line Items]
Non current assets amount	42.5	[4]	37.1
KCPL Greater Missouri Operations Company [Member] | Acquisition transition costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	21.3	[6]	22.5
Regulatory amortization period	2016
KCPL Greater Missouri Operations Company [Member] | St. Joseph Light and Power acquisition [Member]
Regulatory Assets [Line Items]
Non current assets amount	2.2	[8]	2.6
Regulatory amortization period	2015
KCPL Greater Missouri Operations Company [Member] | Storm damage [Member]
Regulatory Assets [Line Items]
Non current assets amount	2.0	[9]	3.2
Regulatory amortization period	2012
KCPL Greater Missouri Operations Company [Member] | Derivative instruments [Member]
Regulatory Assets [Line Items]
Non current assets amount	3.1	[10]	3.1
KCPL Greater Missouri Operations Company [Member] | Iatan No. 1 and Common facilities depreciation and carrying costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	6.1		4.3
KCPL Greater Missouri Operations Company [Member] | Iatan No. 2 construction accounting costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	15.5		6.5
KCPL Greater Missouri Operations Company [Member] | Other [Member]
Regulatory Assets [Line Items]
Non current assets amount	0.4	[7]	0.7
Regulatory amortization period	various
KCPL Greater Missouri Operations Company [Member] | Emission allowances [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	0.3		0.5
KCPL Greater Missouri Operations Company [Member] | Asset retirement obligations [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	0		0
KCPL Greater Missouri Operations Company [Member] | Pension [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	39.9		37.1
KCPL Greater Missouri Operations Company [Member] | Cost of removal, liability [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	62.5	[11]	62.8
KCPL Greater Missouri Operations Company [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	19.5		16.5
Taxes recoverable through future rates [Member]
Regulatory Assets [Line Items]
Non current assets amount	143.9		142.5
Loss on reacquired debt [Member]
Regulatory Assets [Line Items]
Non current assets amount	7.1		5.7
Cost of removal, asset [Member]
Regulatory Assets [Line Items]
Non current assets amount	7.0		8.5
Asset retirement obligations [Member]
Regulatory Assets [Line Items]
Non current assets amount	44.0		40.3
Asset retirement obligations [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	40.9		44.9
Pension and post-retirement costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	493.6		492.8
Deferred customer programs [Member]
Regulatory Assets [Line Items]
Non current assets amount	67.7		60.3
Rate case expenses [Member]
Regulatory Assets [Line Items]
Non current assets amount	14.4		15.6
Skill set realignment costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	3.7		4.8
Fuel adjustment clauses [Member]
Regulatory Assets [Line Items]
Non current assets amount	72.6		45.5
Acquisition transition costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	47.5		51.8
St. Joseph Light and Power acquisition [Member]
Regulatory Assets [Line Items]
Non current assets amount	2.2		2.6
Storm damage [Member]
Regulatory Assets [Line Items]
Non current assets amount	2.0		3.2
Derivative instruments [Member]
Regulatory Assets [Line Items]
Non current assets amount	3.1		3.1
Iatan No. 1 and Common facilities depreciation and carrying costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	22.6		19.4
Iatan No. 2 construction accounting costs [Member]
Regulatory Assets [Line Items]
Non current assets amount	43.1		23.7
Other [Member]
Regulatory Assets [Line Items]
Non current assets amount	2.7		4.2
Emission allowances [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	83.3		86.4
Pension [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	40.4		37.1
Cost of removal, liability [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	62.5		62.8
Other [Member]
Regulatory Liabilities [Line Items]
Non current liabilities	$ 30.7		$ 27.0

[1] 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] Represents the funded status of the pension plans more than offset by related liabilities. Also includes pension settlements amortized over various periods and financial and regulatory accounting method differences not included in rate base that will be eliminated over the life of the pension plans.

[3] $11.3 million not included in rate base and amortized over various periods.

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

[5] $2.5 million not included in rate base and amortized through 2017.

[6] Not included in rate base and amortized through 2016.

[7] Certain insignificant items are not included in rate base and amortized over various periods.

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

[9] Not included in rate base and amortized through 2012.

[10] 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).

[11] Estimated cumulative net provision for future removal costs.