Consolidated Statements of Common Shareholders' Equity and Noncontrolling Interest (unaudited) (USD $) In Millions, except Share data, unless otherwise specified	Total	Kansas City Power and Light Company [Member]	Common Stock [Member]	Common Stock [Member] Kansas City Power and Light Company [Member]	Retained Earnings [Member]	Retained Earnings [Member] Kansas City Power and Light Company [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Other Comprehensive Income (Loss) [Member] Kansas City Power and Light Company [Member]	Noncontrolling Interest [Member]
Beginning balance at Dec. 31, 2010			$ 2,324.4		$ 626.5	$ 478.3	$ (8.9)	$ (56.1)	$ (36.4)	$ 1.2
Beginning balance (in shares) at Dec. 31, 2010			136,113,954				(400,889)
Distribution										0
Total Great Plains Energy Common Shareholders' Equity	2,982.8	2,056.7
Derivative hedging activity, net of tax	4.0	3.9						4.0	3.9
Change in unrecognized pension expense, net of tax	0.1							0.1
Net income (loss) attributable to Great Plains Energy	172.3				172.3	122.8
Loss on reissuance of treasury stock					(0.7)
Treasury shares acquired							(2.3)
Treasury shares acquired (in shares)							(125,024)
Treasury shares reissued							5.6
Treasury shares reissued (in shares)							259,024
Issuance of common stock			4.5
Issuance of common stock (in shares)			226,691
Equity compensation expense, net of forfeitures			0.2
Unearned Compensation [Abstract]
Forfeiture of restricted common stock			0.9
Compensation expense recognized			1.7
Issuance of restricted common stock			(3.5)
Stockholders' Equity, Other			0.3
Dividends:
Common stock					(84.7)	(75.0)
Preferred stock - at required rates					(1.2)
Net income attributable to noncontrolling interest	0
Performance shares					(0.3)
Ending balance at Sep. 30, 2011			2,328.5	1,563.1	711.9	526.1	(5.6)	(52.0)	(32.5)	1.2
Ending balance (in shares) at Sep. 30, 2011			136,340,645	1			(266,889)
Beginning balance at Jun. 30, 2011
Total Great Plains Energy Common Shareholders' Equity	2,982.8	2,056.7
Derivative hedging activity, net of tax	3.1	1.3
Change in unrecognized pension expense, net of tax	0
Net income (loss) attributable to Great Plains Energy	126.5
Dividends:
Net income attributable to noncontrolling interest	0.1
Ending balance at Sep. 30, 2011				1,563.1
Ending balance (in shares) at Sep. 30, 2011				1
Beginning balance at Dec. 31, 2011			2,330.6		684.7	513.8	(5.6)	(49.8)	(31.4)	1.0
Beginning balance (in shares) at Dec. 31, 2011			136,406,306				(264,567)
Distribution										(1.0)
Total Great Plains Energy Common Shareholders' Equity	3,365.4	2,113.0
Derivative hedging activity, net of tax	9.5	4.3						9.5	4.3
Change in unrecognized pension expense, net of tax	0.2							0.2
Net income (loss) attributable to Great Plains Energy	195.2				195.2	136.2
Loss on reissuance of treasury stock					(0.2)
Treasury shares acquired							(3.3)
Treasury shares acquired (in shares)							(164,454)
Treasury shares reissued							3.7
Treasury shares reissued (in shares)							174,795
Issuance of common stock			291.6
Issuance of common stock (in shares)			17,310,715
Equity compensation expense, net of forfeitures			0.2
Unearned Compensation [Abstract]
Forfeiture of restricted common stock			1.3
Compensation expense recognized			1.9
Issuance of restricted common stock			(3.2)
Stockholders' Equity, Other			0.5
Dividends:
Common stock					(90.5)	(73.0)
Preferred stock - at required rates					(1.2)
Net income attributable to noncontrolling interest	0
Performance shares					(0.2)
Ending balance at Sep. 30, 2012			2,622.9	1,563.1	787.8	577.0	(5.2)	(40.1)	(27.1)	0
Ending balance (in shares) at Sep. 30, 2012			153,717,021	1			(254,226)
Beginning balance at Jun. 30, 2012
Total Great Plains Energy Common Shareholders' Equity	3,365.4	2,113.0
Derivative hedging activity, net of tax	3.4	1.7
Change in unrecognized pension expense, net of tax	0.1
Net income (loss) attributable to Great Plains Energy	146.2
Dividends:
Net income attributable to noncontrolling interest	0.2
Ending balance at Sep. 30, 2012				$ 1,563.1
Ending balance (in shares) at Sep. 30, 2012				1