Income Tax Expense (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
Current income taxes
Federal	$ 0	$ (3.3)	$ 0	$ (3.3)
State	0.2	0.4	0.1	0.3
Current Income Tax Expense (Benefit), Total	0.2	(2.9)	0.1	(3.0)
Deferred income taxes
Federal	27.6	30.8	37.5	22.5
State	5.7	5.1	8.0	4.3
Deferred Income Tax Expense (Benefit), Total	33.3	35.9	45.5	26.8
Noncurrent income taxes
Federal	0	(0.2)	0	(0.2)
State	(0.2)	0	(0.2)	0
Foreign	(0.2)	(0.4)	(0.3)	(0.1)
Noncurrent Income Tax Expense (Benefit), Total	(0.4)	(0.6)	(0.5)	(0.3)
Investment tax credit
Amortization	(0.4)	(0.6)	(0.9)	(1.2)
Total income tax expense	32.7	31.8	44.2	22.3
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [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%)	1.20%	0.20%	3.00%
Amortization of investments tax credits (in hundredths)	(0.40%)	(0.70%)	(0.60%)	(1.80%)
Federal income tax credits (in hundredths)	(3.80%)	(3.60%)	(5.00%)	(8.60%)
State income taxes (in hundredths)	3.80%	3.90%	3.90%	4.10%
Changes in uncertain tax positions, net (in hundredths)	(0.30%)	(0.40%)	(0.40%)	(0.10%)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance	0.00%	0.10%	0.00%	0.10%
Effective Income Tax Rate Reconciliation, Other Adjustments	(0.10%)	0.00%	(0.10%)	(0.40%)
Effective Income Tax Rate, Continuing Operations, Total	33.90%	35.50%	33.00%	31.30%
Kansas City Power and Light Company [Member]
Current income taxes
Federal	(0.3)	0.1	(0.6)	0.2
State	(0.1)	0.1	(0.1)	0.1
Current Income Tax Expense (Benefit), Total	(0.4)	0.2	(0.7)	0.3
Deferred income taxes
Federal	16.6	19.4	20.6	16.1
State	3.7	4.2	5.0	4.1
Deferred Income Tax Expense (Benefit), Total	20.3	23.6	25.6	20.2
Noncurrent income taxes
Federal	0.6	(0.3)	1.1	0.1
State	0.1	(0.1)	0.2	0
Noncurrent Income Tax Expense (Benefit), Total	0.7	(0.4)	1.3	0.1
Investment tax credit
Amortization	(0.3)	(0.4)	(0.5)	(0.9)
Total income tax expense	$ 20.3	$ 23.0	$ 25.7	$ 19.7
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [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)	(0.70%)	1.30%	(0.10%)	2.70%
Amortization of investments tax credits (in hundredths)	(0.40%)	(0.70%)	(0.60%)	(1.40%)
Federal income tax credits (in hundredths)	(5.70%)	(4.90%)	(7.70%)	(9.30%)
State income taxes (in hundredths)	3.80%	4.00%	3.80%	4.10%
Changes in uncertain tax positions, net (in hundredths)	(0.10%)	0.00%	(0.10%)	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments	(0.30%)	(0.20%)	(0.40%)	(1.10%)
Effective Income Tax Rate, Continuing Operations, Total	31.60%	34.50%	29.90%	30.00%