Income Tax Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Current income taxes
Federal	$ 0.0	$ (0.1)
State	0.2	0.2
Total	0.2	0.1
Deferred income taxes
Federal	(4.5)	9.7
State	(1.1)	2.3
Total	(5.6)	12.0
Investment tax credit
Investment tax credit amortization	(0.4)	(0.4)
Income tax expense (benefit)	$ (5.8)	$ 11.7
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	35.00%	35.00%
Differences between book and tax depreciation not normalized (in hundredths)	0.00%	0.50%
Amortization of investments tax credits (in hundredths)	(0.40%)	(0.90%)
Federal income tax credits (in hundredths)	(1.50%)	(8.00%)
State income taxes (in hundredths)	4.20%	4.20%
Transaction costs (in hundredths)	3.10%	0.00%
Other (in hundredths)	(2.50%)	0.00%
Effective income tax rate (in hundredths)	37.90%	30.80%
Kansas City Power and Light Company [Member]
Current income taxes
Federal	$ (0.1)	$ 0.4
State	0.0	0.1
Total	(0.1)	0.5
Deferred income taxes
Federal	7.8	8.4
State	1.4	2.0
Total	9.2	10.4
Investment tax credit
Investment tax credit amortization	(0.3)	(0.2)
Income tax expense (benefit)	$ 8.8	$ 10.7
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax (in hundredths)	35.00%	35.00%
Differences between book and tax depreciation not normalized (in hundredths)	(0.10%)	0.40%
Amortization of investments tax credits (in hundredths)	(0.30%)	(0.70%)
Federal income tax credits (in hundredths)	(1.80%)	(7.80%)
State income taxes (in hundredths)	4.00%	3.90%
Other (in hundredths)	1.50%	(0.50%)
Effective income tax rate (in hundredths)	38.30%	30.30%