Fair Value Measurements Level 3 (Details) - Derivative Financial Instruments, Assets [Member] - Fair Value, Inputs, Level 3 [Member] - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net asset (liability) at beginning of period	$ 0.1	$ (1.2)	$ (0.1)	$ 3.5
Purchases	(0.2)	0.4	(0.5)	0.6
Settlements	(2.5)	1.6	(2.6)	1.9
Net asset (liability) at June 30	(76.1)	0.8	(76.1)	0.8
Purchased Power Costs [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	(0.1)	(1.1)	(0.3)	(1.2)
Non-Operating Income [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	2.9	1.6	4.1	3.7
Total unrealized gains and (losses) relating to assets and liabilities still on the consolidated balance sheet at period end	0.1	0.0	0.1	(0.1)
Electric Revenues [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	0.1	(2.5)	(0.3)	(7.7)
Interest Charges [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	(77.0)	0.0	(77.0)	0.0
Total unrealized gains and (losses) relating to assets and liabilities still on the consolidated balance sheet at period end	(77.0)	0.0	(77.0)	0.0
Regulatory Liability [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Regulatory Account	0.6	2.0	0.6	0.0
Total unrealized gains and (losses) relating to assets and liabilities still on the consolidated balance sheet at period end	0.6	0.0	0.6	0.0
Kansas City Power and Light Company [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net asset (liability) at beginning of period	(0.1)	(0.8)	(0.4)	3.1
Purchases	(0.1)	0.2	(0.4)	(0.2)
Settlements	0.1	1.8	1.1	4.9
Net asset (liability) at June 30	0.5	0.1	0.5	0.1
Kansas City Power and Light Company [Member] | Electric Revenues [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	0.1	(2.5)	(0.3)	(7.7)
Kansas City Power and Light Company [Member] | Regulatory Liability [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Regulatory Account	0.5	1.4	0.5	0.0
Total unrealized gains and (losses) relating to assets and liabilities still on the consolidated balance sheet at period end	$ 0.5	$ 0.0	$ 0.5	$ 0.0