REVENUE (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 1,185.1	$ 601.3
Other	31.8	(1.1)
Revenues	1,216.9	600.2
Westar Energy
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	592.3	601.3
Other	4.5	(1.1)
Revenues	596.8	600.2
KCP&L
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	402.9	378.0
Other	22.5	19.1
Revenues	425.4	397.1
Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	451.7	180.3
Residential | Westar Energy
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	192.3	180.3
Residential | KCP&L
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	164.2	154.9
Commercial
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	413.5	155.4
Commercial | Westar Energy
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	164.3	155.4
Commercial | KCP&L
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	183.8	181.8
Industrial
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	147.0	93.5
Industrial | Westar Energy
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	98.4	93.5
Industrial | KCP&L
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	29.7	32.2
Other retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	9.8	4.2
Other retail | Westar Energy
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5.1	4.2
Other retail | KCP&L
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2.6	2.7
Total electric retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,022.0	433.4
Total electric retail | Westar Energy
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	460.1	433.4
Total electric retail | KCP&L
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	380.3	371.6
Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	83.1	94.2
Wholesale | Westar Energy
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	61.3	94.2
Wholesale | KCP&L
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	18.1	3.1
Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	76.7	71.9
Transmission | Westar Energy
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	69.2	71.9
Transmission | KCP&L
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3.1	3.3
Industrial steam and other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3.3	1.8
Industrial steam and other | Westar Energy
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1.7	1.8
Industrial steam and other | KCP&L
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 1.4	$ 0.0