Taxes Uncertain Tax Positions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Uncertain Tax Positions [Line Items]
Amount expected to impact the effective tax rate if recognized	$ 7.3	$ 11.8	$ 17.3
Unrecognized Tax Benefits, Period Increase (Decrease)	(2.6)	(18.0)
Unrecognized tax benefit increase (decrease) related to former GMO non-regulated operations	(4.5)
Unrecognized tax benefit increase (decrease) related to audit settlements		(18.4)
Deferred income tax liability increase (decrease) related to audit settlements		16.4
Increase (decrease) in net income related to settlement of tax audit		2.0
Accrued interest related to unrecognized tax benefits	3.5	5.7	6.7
Accrued penalties with respect to unrecognized tax benefits	0.7	1.1	1.1
Interest expense related to unrecognized tax benefits	2.3	0.9	(0.5)
Amount of unrecognized tax benefits may be recognized in the next twelve months due to statute expirations or settlement agreements with tax authorities	5.5
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	24.0	42.0	51.4
Additions for current year tax positions	3.7	1.4	2.7
Additions for prior year tax positions	0	2.4	2.1
Reductions for prior year tax positions	(1.8)	(20.9)	(10.6)
Settlements	0	0	(3.8)
Statute expirations	(4.7)	(0.7)	(0.3)
Foreign currency translation adjustments	0.2	(0.2)	0.5
Balance at end of period	21.4	24.0	42.0
Kansas City Power and Light Company [Member]
Uncertain Tax Positions [Line Items]
Amount expected to impact the effective tax rate if recognized	0	0.2	0.3
Unrecognized Tax Benefits, Period Increase (Decrease)	1.8	(10.4)
Unrecognized tax benefit increase (decrease) related to audit settlements		(12.1)
Uncertain tax position increase (decrease) related to temporary differences	3.6
Accrued interest related to unrecognized tax benefits	0.1	0.2	1.4
Interest expense related to unrecognized tax benefits	0.1	1.2	0.3
Amount of unrecognized tax benefits may be recognized in the next twelve months due to statute expirations or settlement agreements with tax authorities	4.4
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	8.7	19.1	20.9
Additions for current year tax positions	3.6	0	1.3
Additions for prior year tax positions	0	2.3	1.5
Reductions for prior year tax positions	(1.6)	(12.6)	(1.6)
Settlements	0	0	(2.9)
Statute expirations	(0.2)	(0.1)	(0.1)
Foreign currency translation adjustments	0	0	0
Balance at end of period	$ 10.5	$ 8.7	$ 19.1