Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015		Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]			$ (18.7)		$ (25.3)
Amounts reclassified from accumulated other comprehensive loss	[1]			3.1		5.5
Net current period other comprehensive income		$ 1.6	$ 2.5	3.1	[1]	5.5	[1]
Ending balance	[1]	(15.6)	(19.8)	(15.6)		(19.8)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]			(15.8)		(23.8)
Amounts reclassified from accumulated other comprehensive loss	[1]			2.8		5.3
Net current period other comprehensive income	[1]			2.8		5.3
Ending balance	[1]	(13.0)	(18.5)	(13.0)		(18.5)
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]			(2.9)		(1.5)
Amounts reclassified from accumulated other comprehensive loss	[1]			0.3		0.2
Net current period other comprehensive income	[1]			0.3		0.2
Ending balance	[1]	(2.6)	(1.3)	(2.6)		(1.3)
Kansas City Power and Light Company [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance				(14.9)
Net current period other comprehensive income		1.3	1.4	2.7		2.7
Ending balance		(12.2)		(12.2)
Kansas City Power and Light Company [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]			(14.9)		(20.2)
Amounts reclassified from accumulated other comprehensive loss	[1]			2.7		2.7
Net current period other comprehensive income	[1]			2.7		2.7
Ending balance	[1]	$ (12.2)	$ (17.5)	$ (12.2)		$ (17.5)

[1] Net of tax