Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Kansas City Power and Light Company [Member]	Common Stock [Member]	Common Stock [Member] Kansas City Power and Light Company [Member]	Preferred Stock [Member] Cumulative Preferred Stock [Member]	Preferred Stock [Member] Preference Stock [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]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of ASU 2016-09 (Note 1)								$ 0.0
Beginning balance at Dec. 31, 2015			$ 2,646.7	$ 1,563.1	$ 39.0	$ 0.0	$ 1,024.4	879.6	$ (2.6)	$ (12.0)	$ (9.6)
Beginning balance (in shares) at Dec. 31, 2015			154,504,900	1	390,000	0
Beginning balance, treasury stock (in shares) at Dec. 31, 2015									(101,229)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock			$ 12.5
Issuance of stock (in shares)			420,207
Equity compensation expense, net of forfeitures			$ 3.1
Redemption of preferred stock					$ (39.0)	$ 0.0
Redemption of preferred stock (in shares)					(390,000)	0
Unearned Compensation [Abstract]
Issuance of restricted common stock			(2.8)
Forfeiture of restricted common stock			0.0
Compensation expense recognized			2.0
Other			0.2
Net income (loss)	$ 192.0	$ 208.2					192.0	208.2
Redemption premium on preferred stock							(0.6)
Dividends:
Common stock ($0.825 and $0.7875 per share)							(121.8)	(77.0)
Preferred stock - at required rates							(0.7)
Performance shares							(0.6)
Treasury shares acquired									$ (4.1)
Treasury shares acquired (in shares)									(136,562)
Treasury shares reissued									$ 2.9
Treasury shares reissued (in shares)									109,695
Derivative hedging activity, net of tax	4.1	4.0								4.1	4.0
Change in unrecognized pension expense, net of tax	0.4									0.4
Ending balance at Sep. 30, 2016	3,743.1	2,568.3	$ 2,661.7	$ 1,563.1	$ 0.0	$ 0.0	1,092.7	1,010.8	$ (3.8)	(7.5)	(5.6)
Ending balance, treasury stock (in shares) at Sep. 30, 2016									(128,096)
Ending balance (in shares) at Sep. 30, 2016			154,925,107	1	0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of ASU 2016-09 (Note 1)								(0.7)
Beginning balance at Dec. 31, 2016	$ 6,162.0	2,541.5	$ 4,217.0	$ 1,563.1	$ 0.0	$ 836.2	1,119.2	982.6	$ (3.8)	(6.6)	(4.2)
Beginning balance (in shares) at Dec. 31, 2016			215,479,105	1	0	862,500
Beginning balance, treasury stock (in shares) at Dec. 31, 2016	(128,087)								(128,087)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock			$ 11.6
Issuance of stock (in shares)			319,743
Equity compensation expense, net of forfeitures			$ 4.0
Redemption of preferred stock					$ 0.0	$ (836.2)
Redemption of preferred stock (in shares)					0	(862,500)
Unearned Compensation [Abstract]
Issuance of restricted common stock			(2.3)
Forfeiture of restricted common stock			0.6
Compensation expense recognized			1.7
Other			(1.5)
Net income (loss)	$ (6.1)	177.9					(6.1)	177.9
Redemption premium on preferred stock							(2.4)
Dividends:
Common stock ($0.825 and $0.7875 per share)							(177.8)	(182.0)
Preferred stock - at required rates							(34.9)
Performance shares							(0.4)
Treasury shares acquired									$ (4.2)
Treasury shares acquired (in shares)									(145,301)
Treasury shares reissued									$ 4.0
Treasury shares reissued (in shares)									136,436
Derivative hedging activity, net of tax	4.1	3.8								4.1	3.8
Change in unrecognized pension expense, net of tax	0.3									0.3
Ending balance at Sep. 30, 2017	$ 5,122.5	$ 2,540.5	$ 4,231.1	$ 1,563.1	$ 0.0	$ 0.0	$ 897.6	$ 977.8	$ (4.0)	$ (2.2)	$ (0.4)
Ending balance, treasury stock (in shares) at Sep. 30, 2017	(136,952)								(136,952)
Ending balance (in shares) at Sep. 30, 2017			215,798,848	1	0	0