Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Current income taxes
Federal	$ (30.3)	$ (16.7)	$ (21.5)	$ (14.5)
State	1.5	0.0	1.9	0.4
Total	(28.8)	(16.7)	(19.6)	(14.1)
Deferred income taxes
Federal	70.6	58.8	86.0	101.7
State	23.3	14.3	(35.7)	27.0
Total	93.9	73.1	50.3	128.7
Investment tax credit amortization	(1.0)	(0.6)	(2.4)	(2.0)
Income tax expense	$ 64.1	$ 55.8	$ 28.3	$ 112.6
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	21.00%	35.00%	21.00%	35.00%
COLI policies	(2.40%)	(4.60%)	(2.40%)	(4.50%)
State income taxes	4.60%	4.30%	4.70%	4.30%
Flow through depreciation for plant-related differences	(1.50%)	2.30%	(1.60%)	2.80%
Federal tax credits	(7.50%)	(7.10%)	(7.50%)	(7.00%)
Non-controlling interest	(0.40%)	(0.50%)	(0.40%)	(0.80%)
AFUDC Equity	(0.10%)	(0.20%)	(0.10%)	(0.20%)
Amortization of federal investment tax credits	(0.60%)	(0.60%)	(0.60%)	(0.60%)
State tax rate change	0.10%	0.00%	(9.50%)	0.00%
Valuation allowance	0.00%	0.00%	0.40%	0.00%
Stock compensation	0.00%	0.00%	(0.50%)	(1.00%)
Officer compensation limitation	1.20%	0.00%	1.20%	0.00%
Other	0.80%	(2.80%)	0.40%	(0.70%)
Effective income tax rate (in hundredths)	15.20%	25.80%	5.10%	27.30%
Westar Energy Inc [Member]
Current income taxes
Federal	$ 8.0	$ (16.7)	$ 18.9	$ (14.5)
State	(9.2)	0.0	(6.7)	0.4
Total	(1.2)	(16.7)	12.2	(14.1)
Deferred income taxes
Federal	12.3	58.8	15.4	101.7
State	12.0	14.3	(47.6)	27.0
Total	24.3	73.1	(32.2)	128.7
Investment tax credit amortization	(0.7)	(0.6)	(2.0)	(2.0)
Income tax expense	$ 22.4	$ 55.8	$ (22.0)	$ 112.6
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	21.00%	35.00%	21.00%	35.00%
COLI policies	(4.00%)	(4.60%)	(4.00%)	(4.50%)
State income taxes	1.10%	4.30%	3.30%	4.30%
Flow through depreciation for plant-related differences	1.50%	2.30%	0.30%	2.80%
Federal tax credits	(11.80%)	(7.10%)	(11.70%)	(7.00%)
Non-controlling interest	(0.60%)	(0.50%)	(0.70%)	(0.80%)
AFUDC Equity	(0.10%)	(0.20%)	(0.10%)	(0.20%)
Amortization of federal investment tax credits	(0.80%)	(0.60%)	(0.80%)	(0.60%)
Changes In Uncertain Tax Positions, Net	0.10%	0.00%	0.10%	0.00%
State tax rate change	0.00%	0.00%	(17.70%)	0.00%
Valuation allowance	0.00%	0.00%	0.60%	0.00%
Stock compensation	0.00%	0.00%	(0.90%)	(1.00%)
Officer compensation limitation	1.90%	0.00%	1.90%	0.00%
Other	2.90%	(2.80%)	1.30%	(0.70%)
Effective income tax rate (in hundredths)	11.20%	25.80%	(7.40%)	27.30%
KCPL [Member]
Current income taxes
Federal	$ 20.7	$ 41.8	$ 43.5	$ 56.2
State	6.8	7.6	11.6	10.2
Total	27.5	49.4	55.1	66.4
Deferred income taxes
Federal	(1.9)	10.3	(23.4)	27.2
State	2.7	2.8	49.4	6.2
Total	0.8	13.1	26.0	33.4
Investment tax credit amortization	(0.3)	(0.3)	(0.8)	(0.8)
Income tax expense	$ 28.0	$ 62.2	$ 80.3	$ 99.0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	21.00%	35.00%	21.00%	35.00%
COLI policies	(0.20%)	(0.30%)	(0.20%)	(0.30%)
State income taxes	5.00%	3.90%	5.10%	3.90%
Flow through depreciation for plant-related differences	(4.90%)	0.10%	(4.90%)	0.20%
Federal tax credits	(1.90%)	(3.10%)	(1.80%)	(2.60%)
AFUDC Equity	0.00%	(0.70%)	(0.10%)	(0.70%)
Amortization of federal investment tax credits	(0.40%)	(0.40%)	(0.40%)	(0.40%)
State tax rate change	0.00%	0.00%	14.50%	0.00%
Valuation allowance	0.00%	0.60%	0.00%	0.40%
Stock compensation	0.00%	0.10%	0.00%	0.20%
Officer compensation limitation	0.70%	0.00%	0.50%	0.00%
Other	(0.40%)	0.10%	(1.00%)	0.10%
Effective income tax rate (in hundredths)	18.90%	35.30%	32.70%	35.80%