Accumulated Other Comprehensive Income (Loss) Reclassification (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	$ (49.4)	$ (55.8)	$ (99.1)	$ (122.7)
Fuel	(121.2)	(138.1)	(253.4)	(257.4)
Utility operating and maintenance expenses	(166.4)	(164.1)	(321.6)	(327.2)
Income before income tax (expense) benefit and income (loss) from equity investments	96.4	90.0	134.0	71.2
Income tax (expense) benefit	(32.7)	(31.8)	(44.2)	(22.3)
Net income (loss)	63.6	58.1	89.6	48.8
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss)	(3.1)		(6.3)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income tax (expense) benefit and income (loss) from equity investments	(4.7)		(9.9)
Income tax (expense) benefit	1.8		3.8
Net income (loss)	(2.9)		(6.1)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Interest Rate Contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(4.7)		(9.7)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Commodity Contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Fuel			(0.2)
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Utility operating and maintenance expenses	(0.3)		(0.3)
Income before income tax (expense) benefit and income (loss) from equity investments	(0.3)		(0.3)
Income tax (expense) benefit	0.1		0.1
Net income (loss)	(0.2)		(0.2)
Kansas City Power and Light Company [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(31.5)	(31.3)	(63.5)	(63.7)
Fuel	(86.0)	(98.7)	(180.5)	(183.3)
Utility operating and maintenance expenses	(116.7)	(115.9)	(224.9)	(233.8)
Income before income tax (expense) benefit and income (loss) from equity investments	64.5	66.7	86.1	65.7
Income tax (expense) benefit	(20.3)	(23.0)	(25.7)	(19.7)
Net income (loss)	44.2	43.7	60.4	46.0
Kansas City Power and Light Company [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income tax (expense) benefit and income (loss) from equity investments	(2.2)		(4.6)
Income tax (expense) benefit	0.9		1.8
Net income (loss)	(1.3)		(2.8)
Kansas City Power and Light Company [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Interest Rate Contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(2.2)		(4.4)
Kansas City Power and Light Company [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Commodity Contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Fuel			$ (0.2)