Equity Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 1.8		$ 1.7		$ 2.8		$ 3.3
Income tax benefits	0.7		0.8		1.3		1.4
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility (in hundredths)					21.00%
Expected dividend yield (in hundredths)					4.32%
Risk-free interest rates (in hundredths)					0.40%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Beginning balance (in shares)					442,042
Performance adjustment (in shares)					(160,717)
Granted (in shares)					161,627
Forfeited (in shares)					(50,685)
Ending balance (in shares)	392,267				392,267
Granted and issued (in shares)					161,627
Weighted average beginning balance (in dollars per share)					$ 21.06	[1]
Weighted average granted (in dollars per share)	$ 20.99	[1]	$ 26.28	[1]	$ 19.01	[1]	$ 22.48	[1]
Weighted average forfeited (in dollars per share)	$ 18.71	[1]			$ 18.71	[1]
Weighted average ending balance (in dollars per share)	$ 22.98	[1]			$ 22.98	[1]
Weighted average granted and issued (in dollars per share)	$ 20.99	[1]	$ 26.28	[1]	$ 19.01	[1]	$ 22.48	[1]
Remaining weighted-average contractual term (in years)					1.4
Weighted-average grant-date fair value of shares granted (in dollars per share)	$ 20.99	[1]	$ 26.28	[1]	$ 19.01	[1]	$ 22.48	[1]
Fair value of performance shares earned and paid							0.8
Total unrecognized compensation expense	3.8				3.8
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Beginning balance (in shares)					386,183
Granted (in shares)					162,129
Forfeited (in shares)					(50,685)
Ending balance (in shares)	292,789				292,789
Granted and issued (in shares)					162,129
Vested (in shares)					(204,838)
Weighted average beginning balance (in dollars per share)					$ 17.06	[1]
Weighted average granted (in dollars per share)	$ 19.89	[1]	$ 20.26	[1]	$ 19.69	[1]	$ 19.26	[1]
Weighted average forfeited (in dollars per share)	$ 19.66	[1]			$ 19.66	[1]
Weighted average ending balance (in dollars per share)	$ 18.97	[1]			$ 18.97	[1]
Total fair value of shares vested	1.3		0.6		3.2		2.6
Weighted average granted and issued (in dollars per share)	$ 19.89	[1]	$ 20.26	[1]	$ 19.69	[1]	$ 19.26	[1]
Remaining weighted-average contractual term (in years)					2.0
Weighted-average grant-date fair value of shares granted (in dollars per share)	$ 19.89	[1]	$ 20.26	[1]	$ 19.69	[1]	$ 19.26	[1]
Weighted average vested (in dollars per share)					$ 15.76	[1]
Total unrecognized compensation expense	3.4				3.4
Kansas City Power and Light Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	1.4		1.2		2.1		2.3
Income tax benefits	$ 0.5		$ 0.6		$ 1.0		$ 1.0

[1] weighted-average