Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Federal	$ (1.4)	$ (10.2)	$ (4.7)	$ (6.4)
State	(0.3)	(0.8)	0	(4.8)
Foreign	0	0	0	(0.4)
Current Income Tax Expense (Benefit), Total	(1.7)	(11.0)	(4.7)	(11.6)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Federal	75.1	74.7	97.6	103.6
State	13.9	13.0	18.2	20.4
Deferred Income Tax Expense (Benefit), Total	89.0	87.7	115.8	124.0
Noncurrent income taxes [Abstract]
Federal	0	0	(0.2)	(18.0)
State	0	0	0	(1.9)
Foreign	(4.1)	(0.6)	(4.2)	(0.4)
Noncurrent Income Tax Expense (Benefit), Total	(4.1)	(0.6)	(4.4)	(20.3)
Investment tax credit [Abstract]
Amortization	(0.6)	(0.7)	(1.8)	(1.5)
Total income tax expense	82.6	75.4	104.9	90.6
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Federal statutory income tax	80.0	70.6	105.0	92.0
Differences between book and tax depreciation not normalized	1.3	1.8	3.4	3.7
Amortization of investment tax credits	(0.6)	(0.7)	(1.8)	(1.5)
Federal income tax credits	(2.2)	(2.9)	(8.3)	(9.7)
State income taxes	8.8	8.1	11.7	11.1
Changes in uncertain tax positions, net	(4.1)	(2.7)	(4.2)	(4.0)
Valuation Allowance			0.1	(2.2)
Income Tax Reconciliation, Other Adjustments	(0.6)	1.2	(1.0)	1.2
Total income tax expense	82.6	75.4	104.9	90.6
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)	0.50%	0.90%	1.10%	1.40%
Amortization of investments tax credits (in hundredths)	(0.30%)	(0.30%)	(0.60%)	(0.60%)
Federal income tax credits (in hundredths)	(1.00%)	(1.40%)	(2.80%)	(3.70%)
State income taxes (in hundredths)	3.80%	4.00%	3.90%	4.20%
Changes in uncertain tax positions, net (in hundredths)	(1.80%)	(1.30%)	(1.40%)	(1.50%)
Valuation allowance (in hundredths)			0.00%	(0.80%)
Effective Income Tax Rate Reconciliation, Other Adjustments	(0.10%)	0.40%	(0.30%)	0.50%
Effective Income Tax Rate, Continuing Operations, Total	36.10%	37.30%	34.90%	34.50%
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			2.8		1.4
Additions for prior year tax positions			0		2.4
Reductions for prior year tax positions			(6.1)		(20.9)
Statute expirations			0		(0.7)
Unrecognized Tax Benefits, Increases Resulting from Foreign Currency Translation			0.3		(0.2)
Balance at end of period	21.0		21.0		24.0
Uncertain Tax Positions [Abstract]
Amount expected to impact the effective tax rate if recognized	7.6		7.6		11.8
Unrecognized Tax Benefits, Period Increase (Decrease)			3.0
Increase (decrease) in unrecognized tax benefits related to former GMO non-regulated operations			4.5
Accrued interest related to unrecognized tax benefits	3.5		3.5		5.7
Unrecognized tax benefits, increase (decrease) in interest on income taxes accrued			2.2
Accrued penalties with respect to unrecognized tax benefits	0.7		0.7		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	4.8		4.8
Kansas City Power and Light Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Federal	22.9	(10.2)	23.1	(7.8)
State	4.1	(0.9)	4.2	(0.4)
Current Income Tax Expense (Benefit), Total	27.0	(11.1)	27.3	(8.2)
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Federal	22.7	53.5	38.8	73.4
State	4.5	9.3	8.6	13.8
Deferred Income Tax Expense (Benefit), Total	27.2	62.8	47.4	87.2
Noncurrent income taxes [Abstract]
Federal	1.0	1.5	1.1	(9.1)
State	0.2	0.1	0.2	(1.0)
Noncurrent Income Tax Expense (Benefit), Total	1.2	1.6	1.3	(10.1)
Investment tax credit [Abstract]
Amortization	(0.5)	(0.5)	(1.4)	(1.0)
Total income tax expense	54.9	52.8	74.6	67.9
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Federal statutory income tax	50.8	48.3	73.8	66.7
Differences between book and tax depreciation not normalized	1.0	1.7	2.8	3.3
Amortization of investment tax credits	(0.5)	(0.5)	(1.4)	(1.0)
Federal income tax credits	(2.2)	(2.9)	(8.3)	(9.6)
State income taxes	5.8	5.7	8.4	7.7
Changes in uncertain tax positions, net			0	0.4
Income Tax Reconciliation, Other Adjustments	0	0.5	(0.7)	0.4
Total income tax expense	54.9	52.8	74.6	67.9
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%	1.20%	1.30%	1.70%
Amortization of investments tax credits (in hundredths)	(0.30%)	(0.40%)	(0.60%)	(0.50%)
Federal income tax credits (in hundredths)	(1.50%)	(2.10%)	(4.00%)	(5.10%)
State income taxes (in hundredths)	3.90%	4.10%	4.00%	4.00%
Changes in uncertain tax positions, net (in hundredths)			0.00%	0.20%
Effective Income Tax Rate Reconciliation, Other Adjustments	0.00%	0.40%	(0.30%)	0.30%
Effective Income Tax Rate, Continuing Operations, Total	37.80%	38.20%	35.40%	35.60%
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			2.7		0
Additions for prior year tax positions			0		2.3
Reductions for prior year tax positions			(1.5)		(12.6)
Statute expirations			0		(0.1)
Unrecognized Tax Benefits, Increases Resulting from Foreign Currency Translation			0		0
Balance at end of period	9.9		9.9		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	$ 4.0		$ 4.0