Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Dec. 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	19.3	17.9
Fair Value	(0.6)	(2.8)
Forward Contracts [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	47.7	65.5
Fair Value	5.2	6.5
Option contracts [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	4.8	0
Fair Value	1.2	0
Transmission Congestion Rights [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	22.9	0
Fair Value	1.7	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	7.7	0
Fair Value	(0.2)	0
Kansas City Power and Light Company [Member] | Transmission Congestion Rights [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	18.0	0
Fair Value	$ 1.1	$ 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	40.00%
Percentage of expected on peak natural gas and natural gas equivalent purchased power price exposure hedged year two	6.00%