Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Current income taxes
Federal	$ 0.0	$ 0.0	$ 0.0	$ (0.1)
State	(0.2)	0.1	0.0	0.3
Total	(0.2)	0.1	0.0	0.2
Deferred income taxes
Federal	(7.4)	11.7	(11.9)	21.4
State	(1.4)	3.1	(2.5)	5.4
Total	(8.8)	14.8	(14.4)	26.8
Investment tax credit
Deferral	0.0	2.5	0.0	2.5
Amortization	(0.3)	(0.3)	(0.7)	(0.7)
Total	(0.3)	2.2	(0.7)	1.8
Income tax expense (benefit)	$ (9.3)	$ 17.1	$ (15.1)	$ 28.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	35.00%	35.00%	35.00%	35.00%
Differences between book and tax depreciation not normalized (in hundredths)	0.00%	0.00%	0.00%	0.20%
Amortization of investments tax credits (in hundredths)	(0.80%)	(0.70%)	(0.60%)	(0.80%)
Federal income tax credits (in hundredths)	(3.10%)	(5.90%)	(2.30%)	(6.80%)
State income taxes (in hundredths)	6.30%	4.20%	5.30%	4.20%
Transaction costs (in hundredths)	19.30%	0.00%	11.40%	0.00%
Other (in hundredths)	0.00%	2.10%	(1.20%)	1.20%
Effective income tax rate (in hundredths)	56.70%	34.70%	47.60%	33.00%
Kansas City Power and Light Company [Member]
Current income taxes
Federal	$ 14.5	$ 0.8	$ 14.4	$ 1.2
State	2.6	0.2	2.6	0.3
Total	17.1	1.0	17.0	1.5
Deferred income taxes
Federal	9.1	30.6	16.9	39.0
State	2.0	6.0	3.4	8.0
Total	11.1	36.6	20.3	47.0
Investment tax credit
Amortization	(0.2)	(0.3)	(0.5)	(0.5)
Income tax expense (benefit)	$ 28.0	$ 37.3	$ 36.8	$ 48.0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	35.00%	35.00%	35.00%	35.00%
Differences between book and tax depreciation not normalized (in hundredths)	(0.20%)	(0.10%)	(0.20%)	0.00%
Amortization of investments tax credits (in hundredths)	(0.40%)	(0.30%)	(0.40%)	(0.40%)
Federal income tax credits (in hundredths)	(1.80%)	(2.30%)	(1.80%)	(3.70%)
State income taxes (in hundredths)	3.80%	3.90%	3.90%	3.90%
Other (in hundredths)	(0.30%)	(0.10%)	0.10%	(0.20%)
Effective income tax rate (in hundredths)	36.10%	36.10%	36.60%	34.60%