Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Current income taxes [Abstract]
Federal	$ (3.3)	$ 2.7	$ (3.3)	$ 3.8
State	0.4	(4.7)	0.3	(4.0)
Foreign	0	(0.1)	0	(0.4)
Current Income Tax Expense (Benefit), Total	(2.9)	(2.1)	(3.0)	(0.6)
Deferred income taxes [Abstract]
Federal	30.8	32.3	22.5	28.9
State	5.1	11.4	4.3	7.4
Deferred Income Tax Expense (Benefit), Total	35.9	43.7	26.8	36.3
Noncurrent income taxes [Abstract]
Federal	(0.2)	(19.0)	(0.2)	(18.0)
State	0	(2.0)	0	(1.9)
Foreign	(0.4)	(0.1)	(0.1)	0.2
Noncurrent Income Tax Expense (Benefit), Total	(0.6)	(21.1)	(0.3)	(19.7)
Investment tax credit [Abstract]
Amortization	(0.6)	(0.7)	(1.2)	(0.8)
Total income tax expense	31.8	19.8	22.3	15.2
Reconciliation of income tax expense [Abstract]
Federal statutory income tax	31.5	22.1	25.0	21.4
Differences between book and tax depreciation not normalized	1.0	1.1	2.1	1.9
Amortization of investment tax credits	(0.6)	(0.7)	(1.2)	(0.8)
Federal income tax credits	(3.3)	(3.8)	(6.1)	(6.8)
State income taxes	3.5	2.6	2.9	3.0
Changes in uncertain tax positions, net	(0.3)	(1.6)	(0.1)	(1.3)
Valuation allowance	0.1	0.2	0.1	(2.2)
Income Tax Reconciliation, Other Adjustments	(0.1)	(0.1)	(0.4)	0
Income Tax Expense (Benefit)	31.8	19.8	22.3	15.2
Reconciliation of income tax rate [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)	1.20%	1.60%	3.00%	3.00%
Amortization of investment tax credits (in hundredths)	(0.70%)	(1.10%)	(1.80%)	(1.40%)
Federal income tax credits (in hundredths)	(3.60%)	(6.00%)	(8.60%)	(11.10%)
State income taxes (in hundredths)	3.90%	4.10%	4.10%	4.90%
Changes in uncertain tax positions, net (in hundredths)	(0.40%)	(2.50%)	(0.10%)	(2.20%)
Valuation allowance (in hundredths)	0.10%	0.30%	0.10%	(3.60%)
Effective Income Tax Rate Reconciliation, Other Adjustments	0.00%	0.00%	(0.40%)	0.40%
Effective Income Tax Rate, Continuing Operations, Total	35.50%	31.40%	31.30%	25.00%
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period			24.0	42.0	42.0
Additions for current year tax positions			1.5		1.4
Additions for prior year tax positions			0		2.4
Reductions for prior year tax positions			(1.4)		(20.9)
Statute expirations			0		(0.7)
Foreign currency translation adjustments			(0.1)		(0.2)
Balance at end of period	24.0		24.0		24.0
Uncertain Tax Positions [Abstract]
Amount expected to impact the effective tax rate if recognized	11.7		11.7		11.8
Accrued interest related to unrecognized tax benefits	6.1		6.1		5.7
Accrued penalties with respect to unrecognized tax benefits	1.1		1.1		1.1
Amount of unrecognized tax benefits may be recognized in the next twelve months due to statute expirations or settlement agreements with tax authorities	9.3		9.3
Kansas City Power and Light Company [Member]
Current income taxes [Abstract]
Federal	0.1	1.3	0.2	2.4
State	0.1	0.2	0.1	0.5
Current Income Tax Expense (Benefit), Total	0.2	1.5	0.3	2.9
Deferred income taxes [Abstract]
Federal	19.4	23.2	16.1	19.9
State	4.2	4.7	4.1	4.5
Deferred Income Tax Expense (Benefit), Total	23.6	27.9	20.2	24.4
Noncurrent income taxes [Abstract]
Federal	(0.3)	(11.5)	0.1	(10.6)
State	(0.1)	(1.2)	0	(1.1)
Noncurrent Income Tax Expense (Benefit), Total	(0.4)	(12.7)	0.1	(11.7)
Investment tax credit [Abstract]
Amortization	(0.4)	(0.5)	(0.9)	(0.5)
Total income tax expense	23.0	16.2	19.7	15.1
Reconciliation of income tax expense [Abstract]
Federal statutory income tax	23.4	17.4	23.0	18.4
Differences between book and tax depreciation not normalized	0.9	0.8	1.8	1.6
Amortization of investment tax credits	(0.4)	(0.5)	(0.9)	(0.5)
Federal income tax credits	(3.3)	(3.7)	(6.1)	(6.7)
State income taxes	2.6	1.8	2.6	2.0
Changes in uncertain tax positions, net	0	0.4	0	0.4
Income Tax Reconciliation, Other Adjustments	(0.2)	0	(0.7)	(0.1)
Income Tax Expense (Benefit)	23.0	16.2	19.7	15.1
Reconciliation of income tax rate [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)	1.30%	1.60%	2.70%	3.00%
Amortization of investment tax credits (in hundredths)	(0.70%)	(1.00%)	(1.40%)	(0.90%)
Federal income tax credits (in hundredths)	(4.90%)	(7.60%)	(9.30%)	(12.90%)
State income taxes (in hundredths)	4.00%	3.80%	4.10%	3.90%
Changes in uncertain tax positions, net (in hundredths)	0.00%	0.90%	0.00%	0.80%
Effective Income Tax Rate Reconciliation, Other Adjustments	(0.20%)	(0.20%)	(1.10%)	(0.30%)
Effective Income Tax Rate, Continuing Operations, Total	34.50%	32.50%	30.00%	28.60%
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period			8.7	19.1	19.1
Additions for current year tax positions			1.4		0
Additions for prior year tax positions			0		2.3
Reductions for prior year tax positions			(1.3)		(12.6)
Statute expirations			0		(0.1)
Foreign currency translation adjustments			0		0
Balance at end of period	8.8		8.8		8.7
Uncertain Tax Positions [Abstract]
Amount expected to impact the effective tax rate if recognized	0.2		0.2		0.2
Amount of unrecognized tax benefits may be recognized in the next twelve months due to statute expirations or settlement agreements with tax authorities	$ 4.1		$ 4.1