Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 1,567.0		$ 3,051.4
Other	15.5		24.7
Operating revenues	1,582.5	$ 794.3	3,076.1	$ 1,976.2
Westar Energy Inc [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	761.3		2,011.8
Other	3.5		4.1
Operating revenues	764.8	794.3	2,015.9	1,976.2
KCPL [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	550.8		1,360.2
Other	8.8		48.7
Operating revenues	559.6	$ 595.7	1,408.9	$ 1,474.3
Electric Utility Customer Class Residential [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	647.1		1,169.4
Electric Utility Customer Class Residential [Member] | Westar Energy Inc [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	270.5		671.9
Electric Utility Customer Class Residential [Member] | KCPL [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	243.5		585.4
Electric Utility Customer Class Commercial [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	530.5		945.0
Electric Utility Customer Class Commercial [Member] | Westar Energy Inc [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	217.4		542.8
Electric Utility Customer Class Commercial [Member] | KCPL [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	231.1		609.2
Electric Utility Customer Class Industrial [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	173.4		375.5
Electric Utility Customer Class Industrial [Member] | Westar Energy Inc [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	111.9		297.2
Electric Utility Customer Class Industrial [Member] | KCPL [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	39.1		105.4
Electric Utility Customer Class Other Retail [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	10.9		21.5
Electric Utility Customer Class Other Retail [Member] | Westar Energy Inc [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5.4		15.3
Electric Utility Customer Class Other Retail [Member] | KCPL [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2.7		7.7
Electric Utility Customer Class Total Electric Retail [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,361.9		2,511.4
Electric Utility Customer Class Total Electric Retail [Member] | Westar Energy Inc [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	605.2		1,527.2
Electric Utility Customer Class Total Electric Retail [Member] | KCPL [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	516.4		1,307.7
Electric Utility Customer Class Wholesale [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	118.5		302.4
Electric Utility Customer Class Wholesale [Member] | Westar Energy Inc [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	82.4		263.7
Electric Utility Customer Class Wholesale [Member] | KCPL [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	29.9		38.5
Electric Utility Customer Class Transmission [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	80.6		227.6
Electric Utility Customer Class Transmission [Member] | Westar Energy Inc [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	72.2		216.3
Electric Utility Customer Class Transmission [Member] | KCPL [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3.8		11.0
Electric Utility Customer Class Industrial Steam [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	6.0		10.0
Electric Utility Customer Class Industrial Steam [Member] | Westar Energy Inc [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1.5		4.6
Electric Utility Customer Class Industrial Steam [Member] | KCPL [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 0.7		$ 3.0