Long-Term Debt Long-Term Debt Remarketed (Details) (Kansas City Power and Light Company [Member], USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Secured Debt [Member] | Secured Series 1993A Bonds [Member]
Debt Instrument, Maturity Date	Dec. 31, 2023
Interest rates (in hundredths)	2.95%
EIRR Bonds Remarketed Same Series [Member] | Corporate Debt Securities [Member] | Remarketed EIRR Bonds Series 2007B due 2035 [Member]
EIRR Bonds Remarketed	$ 73.2
Debt Instrument, Maturity Date	Dec. 31, 2035
EIRR Bonds Remarketed Same Series [Member] | Secured Debt [Member] | Secured 1992 Series EIRR Bonds [Member]
EIRR Bonds Remarketed	31.0
Debt Instrument, Maturity Date	Dec. 31, 2017
Interest rates (in hundredths)	1.25%
EIRR Bonds Remarketed Same Series [Member] | Secured Debt [Member] | Secured Series 1993B EIRR Bonds Member
EIRR Bonds Remarketed No Longer Held	39.5
Debt Instrument, Maturity Date	Dec. 31, 2023
Interest rates (in hundredths)	2.95%
EIRR Bonds Remarketed Same Series [Member] | Secured Debt [Member] | Secured Series 1993A Bonds [Member]
EIRR Bonds Remarketed	40.0
EIRR Bonds Remarketed Old Series [Member] | Corporate Debt Securities [Member] | General Mortgage bonds EIRR Series 2007A-1 due 2035 [Member]
EIRR Bonds Remarketed No Longer Held	10.0
EIRR Bonds Remarketed Old Series [Member] | Corporate Debt Securities [Member] | General Mortgage bonds EIRR series 2007A-2 due 2035 [Member]
EIRR Bonds Remarketed No Longer Held	63.3
EIRR Bonds Remarketed New Series [Member] | Corporate Debt Securities [Member] | Remarketed EIRR Bonds Series 2007A due 2035 [Member]
EIRR Bonds Remarketed No Longer Held	73.3
Debt Instrument, Maturity Date	Dec. 31, 2035
Debt retired in connection with EIRR bonds remarketing [Member] | Secured Debt [Member] | Mortgage Bonds Series 2007 EIRR Issuer due 2035 [Member]
Extinguishment of Debt, Amount	$ 146.5