Summary of Significant Accounting Policies Subsequent Events (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Dividends Declared [Abstract]
Subsequent Event Amount	$ 11.8
Subsequent Event Description	In July 2013, the IRS provided guidance to the Company regarding the audit of certain income tax accounting methods for the capitalization of assets. Based on this new guidance, the Company expects to reduce unrecognized tax benefits for these income tax accounting methods by $11.8 million for Great Plains Energy and KCP&L in the third quarter of 2013. This $11.8 million reduction in unrecognized tax benefits will be offset by an increase to deferred income tax liabilities since a significant portion of the unrecognized tax benefits were related to temporary tax differences.
Dividend Declared [Member]
Dividends Declared [Abstract]
Subsequent Event Date	Aug. 06, 2013
Subsequent Event Description	Great Plains Energy’s Board of Directors declared a quarterly dividend on common stock and also declared regular dividends on Great Plains Energy's preferred stock.
Subsequent per common share amount (in dollars per share)	$ 0.2175
Dividend Declared [Member] | Kansas City Power and Light Company [Member]
Dividends Declared [Abstract]
Subsequent Event Date	Aug. 06, 2013
Subsequent Event Amount	$ 23.0
Subsequent Event Description	KCP&L’s Board of Directors declared a cash dividend payable to Great Plains Energy