Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Jan. 02, 2010
Utility plant [Abstract]
Cost	$ 10,924,800,000	[1]	$ 10,536,900,000	[1]
Depreciation and Amortization [Abstract]
Annual depreciation rates (in hundredths)	3.00%
Depreciation expense	239,900,000		243,600,000		228,900,000
Revenue Recognition [Abstract]
Gross receipts taxes collected	55,600,000		54,300,000		46,800,000
Income [Abstract]
Income from continuing operations	174,200,000		211,900,000		151,900,000
Less: net income attributable to noncontrolling interest	(200,000)		200,000		300,000
Less: preferred stock dividend requirements	1,600,000		1,600,000		1,600,000
Income from continuing operations available for common shareholders	172,800,000		210,100,000		150,000,000
Common Shares Outstanding [Abstract]
Average number of common shares outstanding (in shares)	135,600,000		135,100,000		129,300,000
Add: effect of dilutive securities (in shares)	3,100,000		1,800,000		500,000
Diluted average number of common shares outstanding (in shares)	138,700,000		136,900,000		129,800,000
Earnings per share [Abstract]
Basic EPS from continuing operations (in dollars per share)	$ 1.27		$ 1.55		$ 1.16
Diluted EPS from continuing operations (in dollars per share)	$ 1.25		$ 1.53		$ 1.15
Dividends Declared [Abstract]
Subsequent Event Description	Great Plains Energys Board of Directors declared a quarterly dividend and also declared regular dividends on Great Plains Energys preferred stock.
Dividend Declared [Member]
Dividends Declared [Abstract]
Subsequent event date	Feb. 01, 2012
Subsequent per share amount (in dollars per share)	$ 0.2125
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares (in shares)	43,641		251,526		438,281
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares (in shares)	6,000		196,137		231,670
Performance Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive shares (in shares)	50,897		340,690		150,895
Utility plant generation [Member]
Utility plant [Abstract]
Cost	6,594,000,000		6,369,400,000
Useful life minimum	20
Useful life maximum	60
Utiltiy plant transmission [Member]
Utility plant [Abstract]
Cost	734,800,000		716,900,000
Useful life minimum	15
Useful life maximum	70
Utility plant distribution [Member]
Utility plant [Abstract]
Cost	2,921,100,000		2,813,400,000
Useful life minimum	8
Useful life maximum	66
Utility plant general [Member]
Utility plant [Abstract]
Cost	674,900,000		637,200,000
Useful life minimum	5
Useful life maximum	50
Land and other assets [Member]
Utility plant [Abstract]
Cost	105,500,000		103,000,000
Energy Plan [Member]
Depreciation and Amortization [Abstract]
Depreciation expense	14,400,000		72,600,000		58,200,000
Kansas City Power and Light Company [Member]
Utility plant [Abstract]
Cost	7,829,300,000	[2]	7,540,900,000	[2]
Interest rate on allowance for funds used during construction	0.20%		6.80%		7.60%
Depreciation and Amortization [Abstract]
Annual depreciation rates (in hundredths)	3.00%
Depreciation expense	162,000,000		170,900,000		158,400,000
Revenue Recognition [Abstract]
Gross receipts taxes collected	55,600,000		54,300,000		46,800,000
Kansas City Power and Light Company [Member] | Dividend Declared [Member]
Dividends Declared [Abstract]
Subsequent event date	Feb. 01, 2012
Subsequent Event Amount	25,000,000
Subsequent Event Description	KCP&Ls Board of Directors declared a cash dividend payable to Great Plains Energy
Kansas City Power and Light Company [Member] | Utility plant generation [Member]
Utility plant [Abstract]
Cost	5,078,100,000		4,886,200,000
Useful life minimum	20
Useful life maximum	60
Kansas City Power and Light Company [Member] | Utiltiy plant transmission [Member]
Utility plant [Abstract]
Cost	412,900,000		408,700,000
Useful life minimum	15
Useful life maximum	70
Kansas City Power and Light Company [Member] | Utility plant distribution [Member]
Utility plant [Abstract]
Cost	1,840,200,000		1,776,400,000
Useful life minimum	8
Useful life maximum	55
Kansas City Power and Light Company [Member] | Utility plant general [Member]
Utility plant [Abstract]
Cost	498,100,000		469,600,000
Useful life minimum	5
Useful life maximum	50
Kansas City Power and Light Company [Member] | Land and other assets [Member]
Utility plant [Abstract]
Cost	59,800,000		59,900,000
Kansas City Power and Light Company [Member] | Energy Plan [Member]
Depreciation and Amortization [Abstract]
Depreciation expense	14,400,000		72,600,000		58,200,000
KCPL Greater Missouri Operations [Member]
Utility plant [Abstract]
Interest rate on allowance for funds used during construction	5.40%		4.60%		5.40%
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair value of financial instruments [Abstract}
Long-term debt	3,500,000,000		3,400,000,000
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Kansas City Power and Light Company [Member]
Fair value of financial instruments [Abstract}
Long-term debt	1,900,000,000		1,800,000,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair value of financial instruments [Abstract}
Long-term debt	3,900,000,000		3,700,000,000
Estimate of Fair Value, Fair Value Disclosure [Member] | Kansas City Power and Light Company [Member]
Fair value of financial instruments [Abstract}
Long-term debt	$ 2,200,000,000		$ 1,900,000,000

[1] Includes $105.5 million and $103.0 million at December 31, 2011 and 2010, respectively, of land and other assets that are not depreciated.

[2] Includes $59.8 million and $59.9 million at December 31, 2011 and 2010, respectively, of land and other assets that are not depreciated.