SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income
Net income	$ 66.7	$ 370.9	$ 144.6	$ 103.4	$ 21.1	$ 357.6	$ 104.4	$ 62.9	$ 685.6	$ 546.0	$ 336.5
Less: Net income attributable to noncontrolling interests									15.7	10.2	12.6
Net income attributable to Evergy, Inc.	$ 63.9	$ 366.8	$ 139.7	$ 99.5	$ 18.5	$ 355.0	$ 101.8	$ 60.5	$ 669.9	$ 535.8	$ 323.9
Common Shares Outstanding
Weighted average number of common shares outstanding - basic (in shares)									239.5	213.9	142.5
Add: effect of dilutive securities (in shares)									0.4	0.2	0.1
Diluted average number of common shares outstanding (in shares)									239.9	214.1	142.6
Basic EPS (in dollars per share)									$ 2.80	$ 2.50	$ 2.27
Diluted EPS (in dollars per share)									$ 2.79	$ 2.50	$ 2.27