Income Tax Expense (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Current income taxes
Federal	$ (0.7)	$ 1.0	$ (0.6)	$ 1.0
State	(0.1)	(0.4)	0	(0.3)
Current Income Tax Expense (Benefit), Total	(0.8)	0.6	(0.6)	0.7
Deferred income taxes
Federal	69.0	67.9	100.6	105.4
State	12.9	13.3	19.7	21.3
Deferred Income Tax Expense (Benefit), Total	81.9	81.2	120.3	126.7
Noncurrent income taxes
Federal	0	0	(2.4)	0
State	0	0	(0.3)	(0.2)
Foreign	(6.2)	0.1	(6.2)	(0.2)
Noncurrent Income Tax Expense (Benefit), Total	(6.2)	0.1	(8.9)	(0.4)
Investment tax credit
Deferral	0	0.3	0	0.3
Investment tax credit amortization	(0.2)	(0.4)	(1.0)	(1.3)
Investment Tax Credit, Total	(0.2)	(0.1)	(1.0)	(1.0)
Total income tax expense	74.7	81.8	109.8	126.0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	35.00%	35.00%	35.00%	35.00%
Differences between book and tax depreciation not normalized (in hundredths)	(0.30%)	(0.50%)	(0.50%)	(0.20%)
Amortization of investments tax credits (in hundredths)	(0.10%)	(0.20%)	(0.30%)	(0.40%)
Federal income tax credits (in hundredths)	(1.80%)	(1.60%)	(3.00%)	(2.80%)
State income taxes (in hundredths)	3.70%	3.70%	3.70%	3.80%
Changes in uncertain tax positions, net (in hundredths)	(2.80%)	0.10%	(1.80%)	(0.10%)
Effective Income Tax Rate Reconciliation, Other Adjustments	(0.10%)	(0.10%)	(0.20%)	(0.20%)
Effective Income Tax Rate Reconciliation, Percent	33.60%	36.40%	32.90%	35.10%
Kansas City Power and Light Company [Member]
Current income taxes
Federal	33.8	(0.6)	34.5	(1.2)
State	5.9	(0.4)	6.1	(0.5)
Current Income Tax Expense (Benefit), Total	39.7	(1.0)	40.6	(1.7)
Deferred income taxes
Federal	10.9	55.0	26.2	75.6
State	2.9	10.9	6.8	15.9
Deferred Income Tax Expense (Benefit), Total	13.8	65.9	33.0	91.5
Noncurrent income taxes
Federal	0	(10.1)	0	(9.0)
State	0	(1.7)	0	(1.5)
Noncurrent Income Tax Expense (Benefit), Total	0	(11.8)	0	(10.5)
Investment tax credit
Deferral	0	0.3	0	0.3
Investment tax credit amortization	(0.2)	(0.3)	(0.7)	(0.8)
Investment Tax Credit, Total	(0.2)	0	(0.7)	(0.5)
Total income tax expense	$ 53.3	$ 53.1	$ 72.9	$ 78.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	35.00%	35.00%	35.00%	35.00%
Differences between book and tax depreciation not normalized (in hundredths)	(0.20%)	(0.90%)	(0.70%)	(0.60%)
Amortization of investments tax credits (in hundredths)	(0.20%)	(0.20%)	(0.40%)	(0.30%)
Federal income tax credits (in hundredths)	(2.40%)	(2.20%)	(4.30%)	(4.20%)
State income taxes (in hundredths)	3.90%	3.80%	3.80%	3.80%
Changes in uncertain tax positions, net (in hundredths)	0.00%	0.10%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments	0.00%	(0.10%)	(0.10%)	(0.20%)
Effective Income Tax Rate Reconciliation, Percent	36.10%	35.50%	33.30%	33.50%