Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012 Derivative Financial Instruments, Assets [Member]	Jun. 30, 2011 Derivative Financial Instruments, Assets [Member]	Jun. 30, 2012 Derivative Financial Instruments, Assets [Member]	Jun. 30, 2011 Derivative Financial Instruments, Assets [Member]	Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Netting and Collateral [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Netting and Collateral [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Estimate of Fair Value, Fair Value Disclosure [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Estimate of Fair Value, Fair Value Disclosure [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Netting and Collateral [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Netting and Collateral [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Fair Value, Inputs, Level 1 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Fair Value, Inputs, Level 1 [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Fair Value, Inputs, Level 2 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Fair Value, Inputs, Level 2 [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Kansas City Power and Light Company [Member] Fair Value, Inputs, Level 3 [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Estimate of Fair Value, Fair Value Disclosure [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Estimate of Fair Value, Fair Value Disclosure [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Netting and Collateral [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Netting and Collateral [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Fair Value, Inputs, Level 1 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Fair Value, Inputs, Level 1 [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Fair Value, Inputs, Level 2 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Fair Value, Inputs, Level 2 [Member]		Jun. 30, 2012 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Other Great Plains [Member] Fair Value, Inputs, Level 3 [Member]
Assets [Abstract]
Derivative instruments, assets					$ 6.7	[1]	$ 7.8	[1]	$ 0	[2]	$ 0	[2]	$ 0	[1]	$ 0	[1]	$ 4.5	[1]	$ 4.7	[1]	$ 2.2	[1]	$ 3.1	[1]																					$ 6.7	[1]	$ 7.8	[1]	$ 0	[2]	$ 0	[2]	$ 0	[1]	$ 0	[1]	$ 4.5	[1]	$ 4.7	[1]	$ 2.2	[1]	$ 3.1	[1]
Nuclear decommissioning trust [Abstract]
Equity securities related to nuclear decommissioning trust																									91.3		84.3		0	[2]	0	[2]	91.3		84.3		0		0		0		0
Debt securities related to nuclear decommissioning trust [Abstract]
U.S. Treasury																									16.2		15.3		0	[2]	0	[2]	16.2		15.3		0		0		0		0
U.S. Agency																									4.3		3.6		0	[2]	0	[2]	0		0		4.3		3.6		0		0
State and local obligations																									2.7		2.6		0	[2]	0	[2]	0		0		2.7		2.6		0		0
Corporate bonds																									26.9		26.4		0	[2]	0	[2]	0		0		26.9		26.4		0		0
Foreign governments																									0.7		0.7		0	[2]	0	[2]	0		0		0.7		0.7		0		0
Other assets related to nuclear decommissioning trust																									0.5		(0.6)		0	[2]	0	[2]	0		0		0.5		(0.6)		0		0
Total nuclear decommissioning trust					142.6	[3]	132.3	[3]	0	[2]	0	[2]	107.5	[3]	99.6	[3]	35.1	[3]	32.7	[3]	0	[3]	0	[3]	142.6	[3]	132.3	[3]	0	[2]	0	[2]	107.5	[3]	99.6	[3]	35.1	[3]	32.7	[3]	0	[3]	0	[3]
SERP rabbi trust [Abstract]
Equity securities related to SERP rabbi trust																																													0.2		0.2		0	[2]	0	[2]	0.2		0.2		0		0		0		0
Debt securities related to SERP rabbi trust																																													0.1		0.1		0	[2]	0	[2]	0		0		0.1		0.1		0		0
Total SERP rabbi trust					0.3	[4]	0.3	[4]	0	[2]	0	[2]	0.2	[4]	0.2	[4]	0.1	[4]	0.1	[4]	0	[4]	0	[4]																					0.3	[4]	0.3	[4]	0	[2]	0	[2]	0.2	[4]	0.2	[4]	0.1	[4]	0.1	[4]	0	[4]	0	[4]
Total assets					149.6		140.4		0	[2]	0	[2]	107.7		99.8		39.7		37.5		2.2		3.1		142.6		132.3		0	[2]	0	[2]	107.5		99.6		35.1		32.7		0		0		7.0		8.1		0	[2]	0	[2]	0.2		0.2		4.6		4.8		2.2		3.1
Liabilities [Abstract]
Derivative instruments, liabilities					0	[1]	0	[1]	(3.5)	[2]	(5.5)	[2]	3.5	[1]	5.5	[1]	0	[1]	0	[1]	0	[1]	0	[1]	0	[1]	0	[1]	(0.5)	[2]	(0.5)	[2]	0.5	[1]	0.5	[1]	0	[1]	0	[1]	0	[1]	0	[1]	0	[1]	0	[1]	(3.0)	[2]	(5.0)	[2]	3.0	[1]	5.0	[1]	0	[1]	0	[1]	0	[1]	0	[1]
Total liabilities					0		0		(3.5)	[2]	(5.5)	[2]	3.5		5.5		0		0		0		0		0		0		(0.5)	[2]	(0.5)	[2]	0.5		0.5		0		0		0		0		0		0		(3.0)	[2]	(5.0)	[2]	3.0		5.0		0		0		0		0
Cash and cash equivalents from nuclear decommissioning trust excluded from fair value disclosure																									2.3		3.0
Cash and cash equivalents from SERP rabbi trust excluded from fair value disclosure																																													20.2		20.3
Cash collateral posted with third parties as part of netting agreements					3.4		5.5
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	2.4	5.7	3.1	3.7
Total realized/unrealized gains or (losses) included in non-operating income	0.9	2.1	1.1	7.1
Settlements	(1.1)	(2.9)	(2.0)	(5.9)
Balance at end of period	2.2	4.9	2.2	4.9
Total unrealized gains and (losses) included in non-operating income relating to assets and liabilities still on the consolidated balance sheet at period end	$ (0.1)	$ (0.6)	$ (0.7)	$ 1.5

[1] The fair value of derivative instruments is estimated using market quotes, over-the-counter forward price and volatility curves and correlations among fuel prices, net of estimated credit risk.

[2] Represents the difference between derivative contracts in an asset or liability position presented on a net basis by counterparty on the consolidated balance sheet where a master netting agreement exists between the Company and the counterparty. At June 30, 2012, and December 31, 2011, Great Plains Energy netted $3.4 million and $5.5 million, respectively, of cash collateral posted with counterparties.

[3] Fair value is based on quoted market prices of the investments held by the fund and/or valuation models. The total does not include $2.3 million and $3.0 million at June 30, 2012, and December 31, 2011, respectively, of cash and cash equivalents, which are not subject to the fair value requirements.

[4] Fair value is based on quoted market prices of the investments held by the fund and/or valuation models. The total does not include $20.2 million and $20.3 million at June 30, 2012, and December 31, 2011, respectively, of cash and cash equivalents, which are not subject to the fair value requirements.