Derivative Instruments Fair Value (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral			$ 6.5
Derivative liability, fair value, amount not offset against collateral			3.0
Accumulated Other Comprehensive Income (Loss), Net of Tax	(25.3)	[1]	(38.4)	[1]
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(23.8)	[1]	(35.4)	[1]
Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(20.2)		(25.8)
Kansas City Power and Light Company [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(20.2)	[1]	(25.8)	[1]
Current assets [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	9.9		10.6
Current assets [Member] | Kansas City Power and Light Company [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	9.9		10.6
Current liabilities [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	(48.9)		(68.4)
Current liabilities [Member] | Kansas City Power and Light Company [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	(43.1)		(52.8)
Noncurrent liabilities [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	0		(0.1)
Noncurrent liabilities [Member] | Kansas City Power and Light Company [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	0		(0.1)
Deferred income taxes [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	15.2		22.5
Deferred income taxes [Member] | Kansas City Power and Light Company [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Derivatives, Fair Value [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	13.0		16.5
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Other Balance Sheet Location [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral			0
Derivative liability, fair value, amount not offset against collateral			0.2
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Other Balance Sheet Location [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral			0
Derivative liability, fair value, amount not offset against collateral			0.2
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Balance Sheet Location [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral	8.5		6.5
Derivative liability, fair value, amount not offset against collateral	1.0		2.8
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Balance Sheet Location [Member] | Kansas City Power and Light Company [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, amount not offset against collateral	1.2
Derivative liability, fair value, amount not offset against collateral	$ 0.3

[1] Net of tax