Regulatory Matters (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Kansas City Power and Light Company [Member] | Rate jurisdiction - Kansas [Member]
Regulatory Proceedings [Line Items]
Annual revenue increase, requested	$ 63.6
Revised annual revenue increase, requested	56.4
Return on equity, requested (in hundredths)	10.40%
Revised return on equity, requested (in hundredths)	10.30%
Rate-making equity ratio, requested (in hundredths)	51.80%
Annual revenue increase authorized	33.2
Return on equity, approved (in hundredths)	9.50%
Ratemaking equity ratio, approved (in hundredths)	51.80%
Kansas City Power and Light Company [Member] | Rate jurisdiction - Missouri [Member]
Regulatory Proceedings [Line Items]
Annual revenue increase, requested	105.7
Return on equity, requested (in hundredths)	10.40%
Revised return on equity, requested (in hundredths)	10.30%
Rate-making equity ratio, requested (in hundredths)	52.50%
Annual revenue increase authorized	67.4
Return on equity, approved (in hundredths)	9.70%
Ratemaking equity ratio, approved (in hundredths)	52.60%
Rate making equity ratio after including other comprehensive income, approved	52.30%
KCPL Greater Missouri Operations [Member] | Rate jurisdiction - St. Joseph Light and Power division [Member]
Regulatory Proceedings [Line Items]
Annual revenue increase, requested	25.2
Return on equity, requested (in hundredths)	10.40%
Revised return on equity, requested (in hundredths)	10.30%
Rate-making equity ratio, requested (in hundredths)	52.50%
Annual revenue increase authorized	21.7
Return on equity, approved (in hundredths)	9.70%
Ratemaking equity ratio, approved (in hundredths)	52.60%
Rate making equity ratio after including other comprehensive income, approved	52.30%
KCPL Greater Missouri Operations [Member] | Rate jurisdiction - Missouri Public Service division [Member]
Regulatory Proceedings [Line Items]
Annual revenue increase, requested	58.3
Return on equity, requested (in hundredths)	10.40%
Revised return on equity, requested (in hundredths)	10.30%
Rate-making equity ratio, requested (in hundredths)	52.50%
Annual revenue increase authorized	$ 26.2
Return on equity, approved (in hundredths)	9.70%
Ratemaking equity ratio, approved (in hundredths)	52.60%
Rate making equity ratio after including other comprehensive income, approved	52.30%