Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current income taxes
Federal	$ (67.4)	$ 0.1	$ (1.0)
State	2.2	0.4	0.3
Total	(65.2)	0.5	(0.7)
Deferred income taxes
Federal	160.1	122.8	155.2
State	(32.3)	30.7	32.9
Total	127.8	153.5	188.1
Investment Tax Credit
Amortization	(3.6)	(2.8)	(2.9)
Total	(3.6)	(2.8)	(2.9)
Income tax expense	$ 59.0	$ 151.2	$ 184.5
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	21.00%	35.00%	35.00%
COLI policies	(1.90%)	(3.10%)	(4.20%)
State income taxes	4.90%	4.10%	4.00%
Flow through depreciation for plant-related differences	0.80%	2.30%	3.10%
Federal tax credits	(6.40%)	(6.90%)	(1.80%)
Non-controlling interest	(0.40%)	(0.90%)	(0.90%)
AFUDC Equity	(0.10%)	(0.20%)	(0.80%)
Amortization of federal investment tax credits	(0.60%)	(0.60%)	(0.50%)
Changes In Uncertain Tax Positions, Net	0.10%	0.00%	0.00%
Federal or state tax rate change	(8.70%)	2.50%	0.00%
Valuation allowance	0.40%	0.30%	0.40%
Stock compensation	(0.40%)	(0.90%)	(0.50%)
Officer compensation limitation	1.20%	0.20%	0.00%
Other	(0.20%)	(0.80%)	0.00%
Effective income tax rate (in hundredths)	9.70%	31.00%	33.80%
KCPL [Member]
Current income taxes
Federal	$ 29.8	$ 37.4	$ 24.8
State	8.9	8.3	4.7
Total	38.7	45.7	29.5
Deferred income taxes
Federal	(3.4)	74.7	76.4
State	53.0	8.8	17.0
Total	49.6	83.5	93.4
Investment Tax Credit
Amortization	(1.0)	(1.0)	(1.0)
Total	(1.0)	(1.0)	(1.0)
Income tax expense	$ 87.3	$ 128.2	$ 121.9
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	21.00%	35.00%	35.00%
COLI policies	(0.20%)	(0.30%)	(0.20%)
State income taxes	5.50%	3.80%	4.10%
Flow through depreciation for plant-related differences	(2.50%)	0.50%	0.30%
Federal tax credits	(2.10%)	(2.40%)	(3.10%)
AFUDC Equity	(0.10%)	(0.70%)	(0.70%)
Amortization of federal investment tax credits	(0.40%)	(0.30%)	(0.30%)
Federal or state tax rate change	14.10%	5.30%	0.00%
Valuation allowance	0.00%	0.40%	0.00%
Stock compensation	(0.00%)	0.20%	(0.00%)
Officer compensation limitation	0.60%	0.10%	0.20%
Other	(1.00%)	0.00%	(0.10%)
Effective income tax rate (in hundredths)	34.90%	41.60%	35.20%
Westar Energy Inc [Member]
Current income taxes
Federal	$ (0.3)	$ 0.1	$ (1.0)
State	(1.8)	0.4	0.3
Total	(2.1)	0.5	(0.7)
Deferred income taxes
Federal	43.5	122.8	155.2
State	(42.9)	30.7	32.9
Total	0.6	153.5	188.1
Investment Tax Credit
Amortization	(2.8)	(2.8)	(2.9)
Total	(2.8)	(2.8)	(2.9)
Income tax expense	$ (4.3)	$ 151.2	$ 184.5
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	21.00%	35.00%	35.00%
COLI policies	(3.30%)	(3.10%)	(4.20%)
State income taxes	5.00%	4.10%	4.00%
Flow through depreciation for plant-related differences	1.60%	2.30%	3.10%
Federal tax credits	(10.40%)	(6.90%)	(1.80%)
Non-controlling interest	(0.60%)	(0.90%)	(0.90%)
AFUDC Equity	(0.20%)	(0.20%)	(0.80%)
Amortization of federal investment tax credits	(0.80%)	(0.60%)	(0.50%)
Changes In Uncertain Tax Positions, Net	0.10%	0.00%	0.00%
Federal or state tax rate change	(15.30%)	2.50%	0.00%
Valuation allowance	0.50%	0.30%	0.40%
Stock compensation	(0.80%)	(0.90%)	(0.50%)
Officer compensation limitation	1.80%	0.20%	0.00%
Other	0.20%	(0.80%)	0.00%
Effective income tax rate (in hundredths)	(1.20%)	31.00%	33.80%