Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Current income taxes
Federal	$ 0.3	$ (0.1)
State	0.1	(0.1)
Current Income Tax Expense (Benefit), Total	0.4	(0.2)
Deferred income taxes
Federal	6.6	9.9
State	1.7	2.3
Deferred Income Tax Expense (Benefit), Total	8.3	12.2
Noncurrent income taxes
Foreign	(0.2)	(0.1)
Investment tax credit
Investment tax credit amortization	(0.4)	(0.4)
Total income tax expense	8.1	11.5
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	35.00%	35.00%
Differences between book and tax depreciation not normalized (in hundredths)	(1.80%)	1.40%
Amortization of investments tax credits (in hundredths)	(1.20%)	(1.10%)
Federal income tax credits (in hundredths)	(9.10%)	(7.90%)
State income taxes (in hundredths)	3.80%	4.20%
Changes in uncertain tax positions, net (in hundredths)	(0.60%)	(0.70%)
Effective Income Tax Rate Reconciliation, Other Adjustments	(0.70%)	(0.20%)
Effective Income Tax Rate, Continuing Operations, Total	25.40%	30.70%
Kansas City Power and Light Company [Member]
Current income taxes
Federal	2.7	(0.3)
State	0.5	0
Current Income Tax Expense (Benefit), Total	3.2	(0.3)
Deferred income taxes
Federal	0.4	4.0
State	0.6	1.3
Deferred Income Tax Expense (Benefit), Total	1.0	5.3
Noncurrent income taxes
Federal	0	0.6
State	0	0.1
Noncurrent Income Tax Expense (Benefit), Total	0	0.7
Investment tax credit
Investment tax credit amortization	(0.2)	(0.3)
Total income tax expense	$ 4.0	$ 5.4
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	35.00%	35.00%
Differences between book and tax depreciation not normalized (in hundredths)	(3.70%)	1.60%
Amortization of investments tax credits (in hundredths)	(1.20%)	(1.20%)
Federal income tax credits (in hundredths)	(13.80%)	(13.80%)
State income taxes (in hundredths)	3.40%	4.00%
Effective Income Tax Rate Reconciliation, Other Adjustments	(1.10%)	(0.70%)
Effective Income Tax Rate, Continuing Operations, Total	18.60%	24.90%