Consolidated Statements of Changes in Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Retained Earnings [Member]	Noncontrolling Interest [Member]	Westar Energy Inc [Member]	Westar Energy Inc [Member] Common Stock [Member]	Westar Energy Inc [Member] Retained Earnings [Member]	Westar Energy Inc [Member] Noncontrolling Interest [Member]	KCPL [Member]	KCPL [Member] Common Stock [Member]	KCPL [Member] Retained Earnings [Member]	KCPL [Member] AOCI - Net gains (losses) on cash flow hedges [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of ASU 2016-09									$ (0.7)		$ (0.7)
Beginning balance (in shares) at Dec. 31, 2016		141,791,153				141,791,153				1
Beginning balance at Dec. 31, 2016	$ 3,833.2	$ 2,727.3	$ 1,078.6	$ 27.3	$ 3,833.2	$ 2,727.3	$ 1,078.6	$ 27.3	2,541.5	$ 1,563.1	982.6	$ (4.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	300.2		290.0	10.2	300.2		290.0	10.2	177.9		177.9
Issuance of stock	0.6	$ 0.6			0.6	$ 0.6
Issuance of stock (in shares)		12,131				12,131
Issuance of stock for compensation and reinvested dividends (in shares)		290,892				290,892
Issuance of stock for compensation and reinvested dividends	4.9	$ 4.9			4.9	$ 4.9
Tax withholding related to stock compensation	(7.0)	(7.0)			(7.0)	(7.0)
Dividends declared on common stock	(172.1)		(172.1)		(172.1)		(172.1)		(182.0)		(182.0)
Stock compensation expense	6.7	$ 6.7			6.7	$ 6.7
Deconsolidation of noncontrolling interests	(81.9)			(81.9)	(81.9)			(81.9)
Distributions to shareholders of noncontrolling interests	(5.7)			(5.7)	(5.7)			(5.7)
Derivative hedging activity, net of tax									3.8			3.8
Ending balance (in shares) at Sep. 30, 2017		142,094,176				142,094,176				1
Ending balance at Sep. 30, 2017	3,878.9	$ 2,732.5	1,196.5	(50.1)	3,878.9	$ 2,732.5	1,196.5	(50.1)	2,540.5	$ 1,563.1	977.8	(0.4)
Beginning balance (in shares) at Dec. 31, 2017		142,094,275				142,094,275				1
Beginning balance at Dec. 31, 2017	3,860.4	$ 2,734.8	1,173.3	(47.7)	3,860.4	$ 2,734.8	1,173.3	(47.7)	2,513.2	$ 1,563.1	949.7	0.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	524.9		517.3	7.6	318.5		310.9	7.6	165.1		165.1
Issuance of stock to Great Plains Energy shareholders (in shares)		128,947,518
Issuance of stock to Great Plains Energy shareholders	6,979.9	$ 6,979.9
Issuance of restricted common stock (in shares)		122,505
Issuance of stock for compensation and reinvested dividends (in shares)		528,359				516,990
Issuance of stock for compensation and reinvested dividends	0.4	$ 0.4			0.0
Tax withholding related to stock compensation	(17.2)	(17.2)			(17.2)	$ (17.2)
Dividends declared on common stock	(237.5)		(237.5)		(181.5)		(181.5)		(180.0)		(180.0)
Dividend equivalents declared	(0.6)		(0.6)
Stock compensation expense	25.2	25.2			19.9	$ 19.9
Repurchase of common stock	(486.1)	$ (486.1)
Repurchase of common stock		(6,895,073)
Stock cancelled pursuant to Amended Merger Agreement (in shares)						(142,611,264)
Stock cancelled pursuant to Amended Merger Agreement					0.0
Derivative hedging activity, net of tax									2.7			2.7
Other	(0.6)	$ (0.6)			0.1	$ 0.1
Ending balance (in shares) at Sep. 30, 2018		264,797,584				1				1
Ending balance at Sep. 30, 2018	$ 10,648.8	$ 9,236.4	$ 1,452.5	$ (40.1)	$ 4,000.2	$ 2,737.6	$ 1,302.7	$ (40.1)	$ 2,501.0	$ 1,563.1	$ 934.8	$ 3.1