Commitments and Contingencies Commitments and Contingencies Contractual Commitments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Lease Expense [Abstract]
Lease expense	$ 21.8	$ 20.9	$ 17.2
Total Commitments [Abstract]
Total Commitments, Due in Next Twelve Months	847.9
Total Commitments, Due within Two Years	440.6
Total Commitments, Due within Three Years	202.1
Total Commitments, Due within Four Years	141.9
Total Commitments, Due within Five Years	97.4
Total Commitments, Due Thereafter	931.6
Total Commitments Due	2,661.5
Property, Plant Equipment under operating leases [Member]
Operating Leases [Abstract]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	16.8
Operating Leases, Future Minimum Payments, Due in Two Years	14.9
Operating Leases, Future Minimum Payments, Due in Three Years	13.7
Operating Leases, Future Minimum Payments, Due in Four Years	9.9
Operating Leases, Future Minimum Payments, Due in Five Years	9.6
Operating Leases, Future Minimum Payments, Due Thereafter	146.5
Operating Leases, Future Minimum Payments Due, Total	211.4
Property, Plant and Equipment under capital leases [Member]
Capital Leases [Abstract]
Capital Leases, Future Minimum Payments Due, Next Twelve Months	0.4
Capital Leases, Future Minimum Payments Due in Two Years	0.4
Capital Leases, Future Minimum Payments Due in Three Years	0.4
Capital Leases, Future Minimum Payments Due in Four Years	0.4
Capital Leases, Future Minimum Payments Due in Five Years	0.4
Capital Leases, Future Minimum Payments Due Thereafter	4.9
Capital Leases, Future Minimum Payments Due, Total	6.9
Public Utilities, Inventory, Fuel [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	323.2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	209.9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	104.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	66.7
Unrecorded Unconditional Purchase Obligation, Due within Five Years	36.7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	85.4
Unrecorded Unconditional Purchase Obligation, Total	826.8
Purchased Power [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	39.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	44.9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	44.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	44.9
Unrecorded Unconditional Purchase Obligation, Due within Five Years	43.5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	651.5
Unrecorded Unconditional Purchase Obligation, Total	869.0
Capacity [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	11.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	1.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Total	18.8
Non-regulated natural gas transportation [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	3.2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3.6
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3.6
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Total	14.9
La Cygne Environmental Project [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	329.0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	129.6
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Total	463.0
Other purchase commitments [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	124.7
Unrecorded Unconditional Purchase Obligation, Due within Two Years	34.0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	27.2
Unrecorded Unconditional Purchase Obligation, Due within Four Years	15.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	6.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	43.3
Unrecorded Unconditional Purchase Obligation, Total	250.7
Kansas City Power and Light Company [Member]
Lease Expense [Abstract]
Lease expense	17.7	17.0	13.2
Total Commitments [Abstract]
Total Commitments, Due in Next Twelve Months	756.3
Total Commitments, Due within Two Years	391.4
Total Commitments, Due within Three Years	172.9
Total Commitments, Due within Four Years	121.8
Total Commitments, Due within Five Years	86.8
Total Commitments, Due Thereafter	732.8
Total Commitments Due	2,262.0
Kansas City Power and Light Company [Member] | Supply Commitment [Member]
Total Commitments [Abstract]
Capacity Sales Agreements in 2013	5.9
Capacity Sales Agreements per Year (2014 to 2016)	4.3
Kansas City Power and Light Company [Member] | Reimbursement Commitments [Member]
Total Commitments [Abstract]
Reimbursement by Other Owners per Year (2013 to 2015)	2.0
Reimbursements by Other Owners per Year (2016 to 2025)	0.4
Reimbursements by Other Owners Total	10.3
Kansas City Power and Light Company [Member] | Property, Plant Equipment under operating leases [Member]
Operating Leases [Abstract]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	14.5
Operating Leases, Future Minimum Payments, Due in Two Years	13.1
Operating Leases, Future Minimum Payments, Due in Three Years	12.3
Operating Leases, Future Minimum Payments, Due in Four Years	9.8
Operating Leases, Future Minimum Payments, Due in Five Years	9.6
Operating Leases, Future Minimum Payments, Due Thereafter	146.4
Operating Leases, Future Minimum Payments Due, Total	205.7
Kansas City Power and Light Company [Member] | Property, Plant and Equipment under capital leases [Member]
Capital Leases [Abstract]
Capital Leases, Future Minimum Payments Due, Next Twelve Months	0.2
Capital Leases, Future Minimum Payments Due in Two Years	0.2
Capital Leases, Future Minimum Payments Due in Three Years	0.2
Capital Leases, Future Minimum Payments Due in Four Years	0.2
Capital Leases, Future Minimum Payments Due in Five Years	0.2
Capital Leases, Future Minimum Payments Due Thereafter	2.5
Capital Leases, Future Minimum Payments Due, Total	3.5
Kansas City Power and Light Company [Member] | Public Utilities, Inventory, Fuel [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	261.7
Unrecorded Unconditional Purchase Obligation, Due within Two Years	177.6
Unrecorded Unconditional Purchase Obligation, Due within Three Years	91.8
Unrecorded Unconditional Purchase Obligation, Due within Four Years	61.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	36.7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	85.4
Unrecorded Unconditional Purchase Obligation, Total	714.6
Kansas City Power and Light Company [Member] | Purchased Power [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	29.2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	34.8
Unrecorded Unconditional Purchase Obligation, Due within Three Years	34.8
Unrecorded Unconditional Purchase Obligation, Due within Four Years	34.8
Unrecorded Unconditional Purchase Obligation, Due within Five Years	34.8
Unrecorded Unconditional Purchase Obligation, Due after Five Years	464.3
Unrecorded Unconditional Purchase Obligation, Total	632.7
Kansas City Power and Light Company [Member] | Capacity [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	2.9
Unrecorded Unconditional Purchase Obligation, Due within Two Years	2.9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	1.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Total	10.0
Kansas City Power and Light Company [Member] | La Cygne Environmental Project [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	329.0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	129.6
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Total	463.0
Kansas City Power and Light Company [Member] | Other purchase commitments [Member]
Purchase Commitments [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	118.8
Unrecorded Unconditional Purchase Obligation, Due within Two Years	33.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	26.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	14.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5.5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	34.2
Unrecorded Unconditional Purchase Obligation, Total	$ 232.5