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Fair Value Measurements (Details)
$ in Millions
Sep. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Assets [Abstract]      
Derivatives, Interest Rate Swaps, Finalized Settlement Value $ 140.6    
Interest Rate Contract [Member]      
Assets [Abstract]      
Derivative instruments, assets $ 77.4   $ 79.3
Fair Value Inputs, Contingency Factor, Interest Rate Derivative 0.45   0.35
Great Plains Energy [Member]      
Assets [Abstract]      
Derivatives, Interest Rate Swaps, Finalized Settlement Value   $ 140.6  
Fair Value, Measurements, Recurring [Member]      
Assets [Abstract]      
Derivative instruments, assets $ 77.4   $ 79.3
Nuclear decommissioning trust [Abstract]      
Decommissioning Fund Investments, Fair Value 247.5   222.9
Self-insured health plan trust [Abstract]      
Self Insured Health Plan Trust Fair Value Disclosure 11.3   11.3
Total assets 336.2   313.5
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 211.3   185.5
Self-insured health plan trust [Abstract]      
Self Insured Health Plan Trust Fair Value Disclosure 8.9   6.6
Total assets 220.2   192.1
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 36.2   37.4
Self-insured health plan trust [Abstract]      
Self Insured Health Plan Trust Fair Value Disclosure 2.4   4.7
Total assets 38.6   42.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Assets [Abstract]      
Derivative instruments, assets 77.4   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 77.4   79.3
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member]      
Nuclear decommissioning trust [Abstract]      
Decommissioning Fund Investments, Fair Value 247.5   222.9
Self-insured health plan trust [Abstract]      
Self Insured Health Plan Trust Fair Value Disclosure 11.3   11.3
Total assets 258.8   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 174.2   153.9
Self-insured health plan trust [Abstract]      
Self Insured Health Plan Trust Fair Value Disclosure 0.4   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 2.8   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 33.6   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 0.4   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 2.5   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 33.2   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 2.9   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 8.1   5.6
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Other Trust Assets [Member]      
Nuclear decommissioning trust [Abstract]      
Decommissioning Fund Investments, Fair Value 0.6    
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 211.3   185.5
Self-insured health plan trust [Abstract]      
Self Insured Health Plan Trust Fair Value Disclosure 8.9   6.6
Total assets 220.2   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 174.2   153.9
Self-insured health plan trust [Abstract]      
Self Insured Health Plan Trust Fair Value Disclosure 0.4   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 0.4   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 33.6   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 2.9   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 8.1   5.6
Fair Value, Measurements, Recurring [Member] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 1 [Member] | Other Trust Assets [Member]      
Nuclear decommissioning trust [Abstract]      
Decommissioning Fund Investments, Fair Value 0.6    
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 36.2   37.4
Self-insured health plan trust [Abstract]      
Self Insured Health Plan Trust Fair Value Disclosure 2.4   4.7
Total assets 38.6   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 2.4   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 0.4   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 2.5   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 33.2   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 2 [Member] | Other Trust Assets [Member]      
Nuclear decommissioning trust [Abstract]      
Decommissioning Fund Investments, Fair Value 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] | Kansas City Power and Light Company [Member] | Fair Value, Inputs, Level 3 [Member] | Other Trust Assets [Member]      
Nuclear decommissioning trust [Abstract]      
Decommissioning Fund Investments, Fair Value 0.0    
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member]      
Self-insured health plan trust [Abstract]      
Total assets 77.4   79.3
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Interest Rate Contract [Member]      
Assets [Abstract]      
Derivative instruments, assets 77.4   $ 79.3
Derivatives, Interest Rate Swaps, Finalized Settlement Value $ 140.6    
Fair Value Inputs, Contingency Factor, Interest Rate Derivative 0.45   0.35
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
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 2 [Member]      
Self-insured health plan trust [Abstract]      
Total assets 0.0   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 3 [Member]      
Self-insured health plan trust [Abstract]      
Total assets 77.4   79.3
Fair Value, Measurements, Recurring [Member] | Other Consolidated Entities [Member] | Fair Value, Inputs, Level 3 [Member] | Interest Rate Contract [Member]      
Assets [Abstract]      
Derivative instruments, assets $ 77.4   $ 79.3