Long-Term Debt (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Current maturities	$ (257.1)		$ (801.4)
Long-term debt excluding current maturities	2,762.9	[1]	2,742.3	[1]
Long-term debt detail [Abstract]
Subordinated Notes Successfully Remarketed	287.4
Purchase Contracts Settled Early	0.1
Number of outstanding Equity Units at settlement (in millions of units)	5,700,000
Number of common shares issued in settlement of purchase contracts (in millions of shares)	17,100,000
Settlement rate (in shares)	2.9762
Applicable market value (in dollars per share)	$ 20.05
Kansas City Power and Light Company [Member]
Debt Instrument [Line Items]
Current maturities	(0.4)		(12.7)
Unamortized discount	(4.1)		(4.2)
Long-term debt excluding current maturities	1,901.6	[1]	1,901.9	[1]
Kansas City Power and Light Company [Member] | Corporate Debt Securities [Member] | Senior notes due 2018 KCPL [Member]
Debt Instrument [Line Items]
Principal outstanding	350.0		350.0
Debt Instrument, Maturity Date	Dec. 31, 2018
Interest rates (in hundredths)	6.375%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	7.49%	[2]
Kansas City Power and Light Company [Member] | Corporate Debt Securities [Member] | Senior notes due 2035 KCPL [Member]
Debt Instrument [Line Items]
Principal outstanding	250.0		250.0
Debt Instrument, Maturity Date	Dec. 31, 2035
Interest rates (in hundredths)	6.05%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	5.78%	[2]
Kansas City Power and Light Company [Member] | Corporate Debt Securities [Member] | Senior notes due 2017 KCPL [Member]
Debt Instrument [Line Items]
Principal outstanding	250.0		250.0
Debt Instrument, Maturity Date	Dec. 31, 2017
Interest rates (in hundredths)	5.85%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	5.72%	[2]
Kansas City Power and Light Company [Member] | Corporate Debt Securities [Member] | Senior Notes Due 2041 KCPL [Member]
Debt Instrument [Line Items]
Principal outstanding	400.0		400.0
Debt Instrument, Maturity Date	Dec. 31, 2041
Interest rates (in hundredths)	5.30%
Kansas City Power and Light Company [Member] | Other Long-Term Debt [Member]
Debt Instrument [Line Items]
Principal outstanding	2.6		2.9
Debt Instrument Maturity Date Range Start	Dec. 31, 2013
Debt Instrument Maturity Date Range End	Dec. 31, 2018
Kansas City Power and Light Company [Member] | Secured Debt [Member] | General Mortgage bonds EIRR due 2015-2035 [Member]
Debt Instrument [Line Items]
Principal outstanding	106.9		119.3
Debt Instrument Maturity Date Range Start	Dec. 31, 2015
Debt Instrument Maturity Date Range End	Dec. 31, 2035
Interest rates (in hundredths)	4.97%	[3]
Kansas City Power and Light Company [Member] | Secured Debt [Member] | General Mortgage bonds Series 2009 due 2019 [Member]
Debt Instrument [Line Items]
Principal outstanding	400.0		400.0
Debt Instrument, Maturity Date	Dec. 31, 2019
Interest rates (in hundredths)	7.15%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	8.59%	[2]
Kansas City Power and Light Company [Member] | Secured Debt [Member] | General Mortgage bonds EIRR series 2005 due 2035 [Member]
Debt Instrument [Line Items]
Principal outstanding	50.0		50.0
Debt Instrument, Maturity Date	Dec. 31, 2035
Interest rates (in hundredths)	4.65%
Kansas City Power and Light Company [Member] | Secured Debt [Member] | General Mortgage bonds EIRR series 2007B due 2035 [Member]
Debt Instrument [Line Items]
Principal outstanding	73.2		73.2
Debt Instrument, Maturity Date	Dec. 31, 2035
Interest rates (in hundredths)	5.375%
Kansas City Power and Light Company [Member] | Environmental Improvement Revenue Refunding Bonds [Member] | General Mortgage bonds Series 2007A-1 due 2035 [Member]
Debt Instrument [Line Items]
Bonds purchased in lieu of redemption	63.3
Kansas City Power and Light Company [Member] | Environmental Improvement Revenue Refunding Bonds [Member] | 1993 B EIRR Bonds [Member]
Debt Instrument [Line Items]
Bonds purchased in lieu of redemption	39.5
Kansas City Power and Light Company [Member] | Environmental Improvement Revenue Refunding Bonds [Member] | EIRR bonds 4.90 percent Series 2008
Debt Instrument [Line Items]
Principal outstanding	23.4		23.4
Debt Instrument, Maturity Date	Dec. 31, 2038
Interest rates (in hundredths)	4.90%
Kansas City Power and Light Company [Member] | Environmental Improvement Revenue Refunding Bonds [Member] | General Mortgage bonds EIRR series 2007A-2 due 2035 [Member]
Debt Instrument [Line Items]
Bonds purchased in lieu of redemption	10.0
Other Consolidated Entities [Member]
Debt Instrument [Line Items]
Current maturities	(256.7)		(788.7)
Unamortized discount	5.6		(0.7)
Other Consolidated Entities [Member] | GMO First Mortgage Bonds [Member] | First mortgage bonds due 2013-2021 [Member]
Debt Instrument [Line Items]
Principal outstanding	10.1		11.2
Debt Instrument Maturity Date Range Start	Dec. 31, 2013
Debt Instrument Maturity Date Range End	Dec. 31, 2021
Interest rates (in hundredths)	9.44%
Other Consolidated Entities [Member] | GMO Medium Term Notes [Member] | Medium term notes 7.17 percent series due 2023 [Member]
Debt Instrument [Line Items]
Principal outstanding	7.0		7.0
Debt Instrument, Maturity Date	Dec. 31, 2023
Interest rates (in hundredths)	7.17%
Other Consolidated Entities [Member] | GMO Medium Term Notes [Member] | Medium term notes 7.33 percent series due 2023 [Member]
Debt Instrument [Line Items]
Principal outstanding	3.0		3.0
Debt Instrument, Maturity Date	Dec. 31, 2023
Interest rates (in hundredths)	7.33%
Other Consolidated Entities [Member] | GMO Medium Term Notes [Member] | Medium term notes 7.16 percent series due 2013 [Member]
Debt Instrument [Line Items]
Principal outstanding	6.0		6.0
Debt Instrument, Maturity Date	Dec. 31, 2013
Interest rates (in hundredths)	7.16%
Other Consolidated Entities [Member] | GMO Pollution Control Bonds [Member] | Pollution control bonds SJLP due 2013 [Member]
Debt Instrument [Line Items]
Principal outstanding	5.6		5.6
Debt Instrument, Maturity Date	Dec. 31, 2013
Interest rates (in hundredths)	5.85%
Other Consolidated Entities [Member] | GMO Pollution Control Bonds [Member] | Pollution control bonds Wamego Series 1996 due 2026 [Member]
Debt Instrument [Line Items]
Principal outstanding	7.3		7.3
Debt Instrument, Maturity Date	Dec. 31, 2026
Variable rate (in hundredths)	0.318%	[4]
Other Consolidated Entities [Member] | GMO Pollution Control Bonds [Member] | Pollution control bonds state environmental 1993 due 2028 [Member]
Debt Instrument [Line Items]
Principal outstanding	5.0		5.0
Debt Instrument, Maturity Date	Dec. 31, 2028
Variable rate (in hundredths)	0.316%	[4]
Other Consolidated Entities [Member] | GMO Senior Notes [Member] | Senior notes 11.875 percent series due 2012 [Member]
Debt Instrument [Line Items]
Principal outstanding	0		500.0
Debt Instrument, Maturity Date	Dec. 31, 2012
Interest rates (in hundredths)	11.875%
Other Consolidated Entities [Member] | GMO Senior Notes [Member] | Fair value adjustment [Member]
Debt Instrument [Line Items]
Principal outstanding	0		16.3
Other Consolidated Entities [Member] | GMO Senior Notes [Member] | Senior notes 8.27 percent series due 2021 [Member]
Debt Instrument [Line Items]
Principal outstanding	80.9		80.9
Debt Instrument, Maturity Date	Dec. 31, 2021
Interest rates (in hundredths)	8.27%
Other Consolidated Entities [Member] | Great Plains Energy Equity Units Subordinated Notes [Member] | Equity Units Subordinated Notes [Member]
Debt Instrument [Line Items]
Principal outstanding	0		287.5
Interest rates (in hundredths)	10.00%
Other Consolidated Entities [Member] | Great Plains Energy Senior Notes [Member] | Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Principal outstanding	350.0		350.0
Debt Instrument, Maturity Date	Dec. 31, 2021
Interest rates (in hundredths)	4.85%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	7.34%	[2]
Other Consolidated Entities [Member] | Great Plains Energy Senior Notes [Member] | Senor notes due 2013 [Member]
Debt Instrument [Line Items]
Principal outstanding	250.0		250.0
Debt Instrument, Maturity Date	Dec. 31, 2013
Interest rates (in hundredths)	2.75%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	3.67%	[2]
Other Consolidated Entities [Member] | Great Plains Energy Senior Notes [Member] | Senior notes due 2017 [Member]
Debt Instrument [Line Items]
Principal outstanding	100.0		100.0
Debt Instrument, Maturity Date	Dec. 31, 2017
Interest rates (in hundredths)	6.875%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	7.33%	[2]
Other Consolidated Entities [Member] | Great Plains Energy Senior Notes [Member] | Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Principal outstanding	$ 287.5		$ 0
Debt Instrument, Maturity Date	Dec. 31, 2022
Interest rates (in hundredths)	5.292%

[1] Does not include $39.5 million EIRR Series 1993B, $63.3 million EIRR Series 2007 A-1 and $10.0 million EIRR Series 2007 A-2 bonds because the bonds have been repurchased and are held by KCP&L

[2] Rate after amortizing gains/losses recognized in OCI on settlements of interest rate hedging instruments

[3] Weighted-average interest rates at September 30, 2012

[4] Variable rate