Fair Value Measurements (Details) $ in Millions	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Assets [Abstract]
Derivatives, Interest Rate Swaps, Finalized Settlement Value	$ 140.6
Interest Rate Contract [Member]
Assets [Abstract]
Derivative instruments, assets	$ 49.2	$ 79.3
Fair Value Inputs, Contingency Factor, Interest Rate Derivative	0.65	0.35
Fair Value, Measurements, Recurring [Member]
Assets [Abstract]
Derivative instruments, assets	$ 49.2	$ 79.3
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	238.4	222.9
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	11.4	11.3
Total assets	299.0	313.5
Liabilities [Abstract]
Derivative instruments, liabilities	57.1
Total liabilities	57.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets [Abstract]
Derivative instruments, assets	0.0	0.0
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	201.6	185.5
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	8.9	6.6
Total assets	210.5	192.1
Liabilities [Abstract]
Derivative instruments, liabilities	0.0
Total liabilities	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets [Abstract]
Derivative instruments, assets	0.0	0.0
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	36.8	37.4
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	2.5	4.7
Total assets	39.3	42.1
Liabilities [Abstract]
Derivative instruments, liabilities	0.0
Total liabilities	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets [Abstract]
Derivative instruments, assets	49.2	79.3
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Total assets	49.2	79.3
Liabilities [Abstract]
Derivative instruments, liabilities	57.1
Total liabilities	57.1
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	238.4	222.9
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	11.4	11.3
Total assets	249.8	234.2
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Equity Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	165.8	153.9
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.7	0.9
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Debt Securities [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	2.5	4.8
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | US Treasury and Government [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	32.7	27.8
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | US Government Corporations and Agencies Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	1.1	1.7
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | US States and Political Subdivisions Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	4.1	3.2
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Corporate Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	31.5	32.4
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Foreign Government Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.1	0.1
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Cash Equivalents [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	3.1	3.8
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Cash and Cash Equivalents [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	8.2	5.6
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	201.6	185.5
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	8.9	6.6
Total assets	210.5	192.1
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	165.8	153.9
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.7	0.9
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Debt Securities [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.1
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury and Government [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	32.7	27.8
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Corporations and Agencies Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign Government Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	3.1	3.8
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	8.2	5.6
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	36.8	37.4
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	2.5	4.7
Total assets	39.3	42.1
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	2.5	4.7
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury and Government [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | US Government Corporations and Agencies Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	1.1	1.7
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	4.1	3.2
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	31.5	32.4
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Government Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.1	0.1
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 2 [Member] | Cash and Cash Equivalents [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Total assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | US Treasury and Government [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | US Government Corporations and Agencies Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | US States and Political Subdivisions Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign Government Debt Securities [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Cash Equivalents [Member]
Nuclear decommissioning trust [Abstract]
Decommissioning Fund Investments, Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Cash and Cash Equivalents [Member]
Self-insured health plan trust [Abstract]
Self Insured Health Plan Trust Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member]
Self-insured health plan trust [Abstract]
Total assets	49.2	79.3
Liabilities [Abstract]
Total liabilities	57.1
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Interest Rate Contract [Member]
Assets [Abstract]
Derivative instruments, assets	49.2	$ 79.3
Derivatives, Interest Rate Swaps, Finalized Settlement Value	$ 140.6
Fair Value Inputs, Contingency Factor, Interest Rate Derivative	0.65	0.35
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Dividend Make-Whole Provision [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	$ 57.1
Fair Value Inputs, Contingency Factor, Make-Whole Provisions	0.53
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 1 [Member]
Self-insured health plan trust [Abstract]
Total assets	$ 0.0	$ 0.0
Liabilities [Abstract]
Total liabilities	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 1 [Member] | Interest Rate Contract [Member]
Assets [Abstract]
Derivative instruments, assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 1 [Member] | Dividend Make-Whole Provision [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 2 [Member]
Self-insured health plan trust [Abstract]
Total assets	0.0	0.0
Liabilities [Abstract]
Total liabilities	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 2 [Member] | Interest Rate Contract [Member]
Assets [Abstract]
Derivative instruments, assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 2 [Member] | Dividend Make-Whole Provision [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	0.0
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 3 [Member]
Self-insured health plan trust [Abstract]
Total assets	49.2	79.3
Liabilities [Abstract]
Total liabilities	57.1
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 3 [Member] | Interest Rate Contract [Member]
Assets [Abstract]
Derivative instruments, assets	49.2	$ 79.3
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 3 [Member] | Dividend Make-Whole Provision [Member]
Liabilities [Abstract]
Derivative instruments, liabilities	$ 57.1