Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Futures contracts [Member] \| Cash Flow Hedging [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	$ 0	$ 1.0
Fair Value	0	(0.2)
Futures contracts [Member] \| Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	38.5	17.9
Fair Value	(2.0)	(2.8)
Forward Contracts [Member] \| Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	67.0	65.5
Fair Value	6.5	6.5
Option contracts [Member] \| Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	2.5	0
Fair Value	0.5	0
Kansas City Power and Light Company [Member] \| Futures contracts [Member] \| Cash Flow Hedging [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	0	1.0
Fair Value	0	(0.2)
Kansas City Power and Light Company [Member] \| Futures contracts [Member] \| Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	20.3	0
Fair Value	$ (1.0)	$ 0
KCPL Greater Missouri Operations [Member]
Price Risk Derivatives [Abstract]
Percentage of expected on peak natural gas and natural gas equivalent purchased power price exposure hedged year one	83.00%
Percentage of expected on peak natural gas and natural gas equivalent purchased power price exposure hedged year two	22.00%
Percentage of expected on peak natural gas and natural gas equivalent purchased power price exposure hedged year three	2.00%

KANSAS CITY POWER & LIGHT CO 10-Q