Short-term Borrowings and Short-term Bank Lines of Credit (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Short-term Debt [Line Items]
Amount of outstanding cash borrowings	$ 31.0	$ 22.0
Commercial paper outstanding	427.0	267.0
Parent Company [Member]
Short-term Debt [Line Items]
Amount of revolving credit facility	200
Revolving credit facility expiration date	Dec. 01, 2016
Covenant term - amount considered in default	50.0
Amount of outstanding cash borrowings	31.0	22.0
Weighted-average interest rate from outstanding borrowings	2.00%	2.06%
Amount of letters of credit outstanding	1.8	11.6
Covenant Term Ratio of Consolidated Indebtedness to Consolidated Capitalization Ratio, Maximum	0.65
Line of Credit Facility, Covenant Compliance	in compliance
Kansas City Power and Light Company [Member]
Short-term Debt [Line Items]
Amount of revolving credit facility	600
Revolving credit facility expiration date	Dec. 01, 2016
Covenant term - amount considered in default	50.0
Amount of outstanding cash borrowings	0	0
Commercial paper outstanding	256.0	227.0
Weighted-average interest rate from outstanding borrowings	0.49%	0.50%
Amount of letters of credit outstanding	13.9	21.5
Covenant Term Ratio of Consolidated Indebtedness to Consolidated Capitalization Ratio, Maximum	0.65
Line of Credit Facility, Covenant Compliance	in compliance
KCPL Greater Missouri Operations [Member]
Short-term Debt [Line Items]
Amount of revolving credit facility	450
Revolving credit facility expiration date	Dec. 01, 2016
Covenant term - amount considered in default	50.0
Amount of outstanding cash borrowings	0	0
Commercial paper outstanding	171.0	40.0
Weighted-average interest rate from outstanding borrowings	0.94%	0.88%
Amount of letters of credit outstanding	$ 15.1	$ 13.2
Covenant Term Ratio of Consolidated Indebtedness to Consolidated Capitalization Ratio, Maximum	0.65
Line of Credit Facility, Covenant Compliance	in compliance