Derivative Instruments, Group 2 (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012 Kansas City Power and Light Company [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Derivative Instrument, Current and Noncurrent [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Derivative Instrument, Current and Noncurrent [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Derivative Instrument, Current and Noncurrent [Member] Kansas City Power and Light Company [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Derivative Instrument, Current and Noncurrent [Member] Kansas City Power and Light Company [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Current assets [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Current assets [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Current assets [Member] Kansas City Power and Light Company [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Current assets [Member] Kansas City Power and Light Company [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Current liabilities [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Current liabilities [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Current liabilities [Member] Kansas City Power and Light Company [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Current liabilities [Member] Kansas City Power and Light Company [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Noncurrent liabilities [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Noncurrent liabilities [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Noncurrent liabilities [Member] Kansas City Power and Light Company [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Noncurrent liabilities [Member] Kansas City Power and Light Company [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Deferred income taxes [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Deferred income taxes [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Deferred income taxes [Member] Kansas City Power and Light Company [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Deferred income taxes [Member] Kansas City Power and Light Company [Member]	Jun. 30, 2012 Commodity Contract [Member] Designated as Hedging Instrument [Member] Derivative Instrument, Current and Noncurrent [Member]	Dec. 31, 2011 Commodity Contract [Member] Designated as Hedging Instrument [Member] Derivative Instrument, Current and Noncurrent [Member]	Jun. 30, 2012 Commodity Contract [Member] Designated as Hedging Instrument [Member] Derivative Instrument, Current and Noncurrent [Member] Kansas City Power and Light Company [Member]	Dec. 31, 2011 Commodity Contract [Member] Designated as Hedging Instrument [Member] Derivative Instrument, Current and Noncurrent [Member] Kansas City Power and Light Company [Member]	Jun. 30, 2012 Commodity Contract [Member] Not Designated as Hedging Instrument [Member] Derivative Instrument, Current and Noncurrent [Member]	Dec. 31, 2011 Commodity Contract [Member] Not Designated as Hedging Instrument [Member] Derivative Instrument, Current and Noncurrent [Member]	Jun. 30, 2012 Total Derivatives [Member] Derivative Instrument, Current and Noncurrent [Member]	Dec. 31, 2011 Total Derivatives [Member] Derivative Instrument, Current and Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral																							$ 0	$ 0	$ 0	$ 0	$ 6.7	$ 7.8	$ 6.7	$ 7.8
Derivative liability, fair value, amount not offset against collateral																							0.5	0.5	0.5	0.5	3.0	5.0	3.5	5.5
Recorded amounts included in accumulated other comprehensive income			(41.8)	(47.9)	(28.8)	(31.4)	10.9	11.3	10.9	11.3	(79.1)	(89.5)	(57.8)	(62.5)	(0.3)	(0.2)	(0.3)	(0.2)	26.7	30.5	18.4	20.0
Derivative [Line Items]
Accumulated other comprehensive income that is expected to be reclassified in the next twelve months	$ 20.6	$ 9.1