Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Futures contracts [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	$ 28.8	$ 19.3
Fair Value	(0.1)	(0.6)
Forward Contracts [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	49.2	47.7
Fair Value	5.2	5.2
Option contracts [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	3.1	4.8
Fair Value	1.0	1.2
Transmission Congestion Rights [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	24.1	22.9
Fair Value	5.3	1.7
Kansas City Power and Light Company [Member] | Futures contracts [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	16.4	7.7
Fair Value	(0.3)	(0.2)
Kansas City Power and Light Company [Member] | Transmission Congestion Rights [Member] | Not Designated as Hedging Instrument [Member]
Price Risk Derivatives [Abstract]
Notional Contract Amount	19.0	18.0
Fair Value	$ 3.9	$ 1.1
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	43.00%
Percentage of expected on peak natural gas and natural gas equivalent purchased power price exposure hedged year two	12.00%