Long-Term Debt (Details) (USD $)	6 Months Ended
	Jun. 30, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Current maturities	$ 507,100,000		$ 801,400,000
Long-term debt excluding current maturities	3,013,400,000	[1]	2,742,300,000	[1]
Long-term debt detail [Abstract]
Subordinated Notes Successfully Remarketed	287,400,000
Purchase Contracts Settled Early	100,000
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
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair value of financial instruments [Abstract}
Long-term debt	3,500,000,000		3,500,000,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair value of financial instruments [Abstract}
Long-term debt	3,900,000,000		3,900,000,000
Kansas City Power and Light Company [Member]
Debt Instrument [Line Items]
Current maturities	400,000		12,700,000
Unamortized discount	(4,000,000)		(4,200,000)
Long-term debt excluding current maturities	1,902,000,000	[1]	1,901,900,000	[1]
Kansas City Power and Light Company [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair value of financial instruments [Abstract}
Long-term debt	1,900,000,000		1,900,000,000
Kansas City Power and Light Company [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair value of financial instruments [Abstract}
Long-term debt	2,200,000,000		2,200,000,000
Kansas City Power and Light Company [Member] | Corporate Debt Securities [Member] | Senior notes due 2018 KCPL [Member]
Debt Instrument [Line Items]
Principal outstanding	350,000,000		350,000,000
Year due	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	250,000,000		250,000,000
Year due	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	250,000,000		250,000,000
Year due	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	400,000,000		400,000,000
Year due	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	2,900,000		2,900,000
Debt Instrument Maturity Date Range Start	2012
Debt Instrument Maturity Date Range End	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	106,900,000		119,300,000
Debt Instrument Maturity Date Range Start	2015
Debt Instrument Maturity Date Range End	2035
Weighted-average 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	400,000,000		400,000,000
Year due	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	50,000,000		50,000,000
Year due	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	73,200,000		73,200,000
Year due	2035
Interest rates (in hundredths)	5.375%
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	10,000,000		10,000,000
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	63,300,000		63,300,000
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	39,500,000		39,500,000
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	23,400,000		23,400,000
Year due	2038
Interest rates (in hundredths)	4.90%
Other Great Plains [Member]
Debt Instrument [Line Items]
Current maturities	(506,700,000)		(788,700,000)
Unamortized discount	5,700,000		(700,000)
Other Great Plains [Member] | GMO First Mortgage Bonds [Member] | First mortgage bonds due 2013-2021 [Member]
Debt Instrument [Line Items]
Principal outstanding	10,100,000		11,200,000
Debt Instrument Maturity Date Range Start	2013
Debt Instrument Maturity Date Range End	2021
Interest rates (in hundredths)	9.44%
Other Great Plains [Member] | GMO Medium Term Notes [Member] | Medium term notes 7.17 percent series due 2023 [Member]
Debt Instrument [Line Items]
Principal outstanding	7,000,000		7,000,000
Year due	2023
Interest rates (in hundredths)	7.17%
Other Great Plains [Member] | GMO Medium Term Notes [Member] | Medium term notes 7.33 percent series due 2023 [Member]
Debt Instrument [Line Items]
Principal outstanding	3,000,000		3,000,000
Year due	2023
Interest rates (in hundredths)	7.33%
Other Great Plains [Member] | GMO Medium Term Notes [Member] | Medium term notes 7.16 percent series due 2013 [Member]
Debt Instrument [Line Items]
Principal outstanding	6,000,000		6,000,000
Year due	2013
Interest rates (in hundredths)	7.16%
Other Great Plains [Member] | GMO Pollution Control Bonds [Member] | Pollution control bonds SJLP due 2013 [Member]
Debt Instrument [Line Items]
Principal outstanding	5,600,000		5,600,000
Year due	2013
Interest rates (in hundredths)	5.85%
Other Great Plains [Member] | GMO Pollution Control Bonds [Member] | Pollution control bonds Wamego Series 1996 due 2026 [Member]
Debt Instrument [Line Items]
Principal outstanding	7,300,000		7,300,000
Year due	2026
Variable rate (in hundredths)	0.341%	[4]
Other Great Plains [Member] | GMO Pollution Control Bonds [Member] | Pollution control bonds state environmental 1993 due 2028 [Member]
Debt Instrument [Line Items]
Principal outstanding	5,000,000		5,000,000
Year due	2028
Variable rate (in hundredths)	0.341%	[4]
Other Great Plains [Member] | GMO Senior Notes [Member] | Senior notes 11.875 percent series due 2012 [Member]
Debt Instrument [Line Items]
Principal outstanding	500,000,000		500,000,000
Year due	2012
Interest rates (in hundredths)	11.875%
Other Great Plains [Member] | GMO Senior Notes [Member] | Fair value adjustment [Member]
Debt Instrument [Line Items]
Principal outstanding	0		16,300,000
Other Great Plains [Member] | GMO Senior Notes [Member] | Senior notes 8.27 percent series due 2021 [Member]
Debt Instrument [Line Items]
Principal outstanding	80,900,000		80,900,000
Year due	2021
Interest rates (in hundredths)	8.27%
Other Great Plains [Member] | Great Plains Energy Equity Units Subordinated Notes [Member] | Equity Units Subordinated Notes [Member]
Debt Instrument [Line Items]
Principal outstanding	0		287,500,000
Interest rates (in hundredths)	10.00%
Other Great Plains [Member] | Great Plains Energy Senior Notes [Member] | Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Principal outstanding	350,000,000		350,000,000
Year due	2021
Interest rates (in hundredths)	4.85%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	7.34%	[2]
Other Great Plains [Member] | Great Plains Energy Senior Notes [Member] | Senor notes due 2013 [Member]
Debt Instrument [Line Items]
Principal outstanding	250,000,000		250,000,000
Year due	2013
Interest rates (in hundredths)	2.75%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	3.67%	[2]
Other Great Plains [Member] | Great Plains Energy Senior Notes [Member] | Senior notes due 2017 [Member]
Debt Instrument [Line Items]
Principal outstanding	100,000,000		100,000,000
Year due	2017
Interest rates (in hundredths)	6.875%
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)	7.33%	[2]
Other Great Plains [Member] | Great Plains Energy Senior Notes [Member] | Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Principal outstanding	$ 287,500,000		$ 0
Year due	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 June 30, 2012

[4] Variable rate