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Long-Term Debt (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Jan. 02, 2010
Dec. 31, 2011
$16.80 or greater [Member]
Dec. 31, 2011
$16.80 to $14.00 [Member]
Dec. 31, 2011
$14.00 or less [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Jan. 02, 2010
Kansas City Power and Light Company [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Secured 1992 Series EIRR Bonds [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Secured Series 1993A and 1993B EIRR bonds [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Secured EIRR Bonds Series 2005 [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior notes due 2018 KCPL [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior notes due 2018 KCPL [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior notes due 2035 KCPL [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior notes due 2035 KCPL [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior notes due 2011 KCPL [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior notes due 2011 KCPL [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior notes due 2017 [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior notes due 2017 [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior Notes Due 2041 KCPL [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Corporate Debt Securities [Member]
Senior Notes Due 2041 KCPL [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Other Long-Term Debt [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Other Long-Term Debt [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds EIRR due 2012-2035 [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds EIRR due 2012-2035 [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds Series 2009 due 2019 [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds Series 2009 due 2019 [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds EIRR series 2005 due 2035 [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds EIRR series 2005 due 2035 [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds EIRR series 2007A-2 due 2035 [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds EIRR series 2007A-2 due 2035 [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds EIRR series 2007B due 2035 [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds EIRR series 2007B due 2035 [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds Series 2007A-1 due 2035 [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Secured Debt [Member]
General Mortgage bonds Series 2007A-1 due 2035 [Member]
Dec. 31, 2011
Kansas City Power and Light Company [Member]
Environmental Improvement Revenue Refunding Bonds [Member]
Dec. 31, 2010
Kansas City Power and Light Company [Member]
Environmental Improvement Revenue Refunding Bonds [Member]
Dec. 31, 2011
Other Great Plains [Member]
Dec. 31, 2010
Other Great Plains [Member]
Jan. 02, 2010
Other Great Plains [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO First Mortgage Bonds [Member]
First mortgage bonds due 2012-2021 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO First Mortgage Bonds [Member]
First mortgage bonds due 2012-2021 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Medium Term Notes [Member]
Medium term notes 7.17 percent series due 2023 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Medium Term Notes [Member]
Medium term notes 7.17 percent series due 2023 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Medium Term Notes [Member]
Medium term notes 7.33 percent series due 2023 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Medium Term Notes [Member]
Medium term notes 7.33 percent series due 2023 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Medium Term Notes [Member]
Medium term notes 7.16 percent series due 2013 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Medium Term Notes [Member]
Medium term notes 7.16 percent series due 2013 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Pollution Control Bonds [Member]
Pollution control bonds SJLP due 2013 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Pollution Control Bonds [Member]
Pollution control bonds SJLP due 2013 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Pollution Control Bonds [Member]
Pollution control bonds Wamego Series 1996 due 2026 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Pollution Control Bonds [Member]
Pollution control bonds Wamego Series 1996 due 2026 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Pollution Control Bonds [Member]
Pollution control bonds state environmental 1993 due 2028 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Pollution Control Bonds [Member]
Pollution control bonds state environmental 1993 due 2028 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Senior Notes [Member]
Senior notes 11.875 percent series due 2012 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Senior Notes [Member]
Senior notes 11.875 percent series due 2012 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Senior Notes [Member]
Senior notes 7.75 percent series due 2011 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Senior Notes [Member]
Senior notes 7.75 percent series due 2011 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Senior Notes [Member]
Fair value adjustment [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Senior Notes [Member]
Fair value adjustment [Member]
Jan. 02, 2010
Other Great Plains [Member]
GMO Senior Notes [Member]
Fair value adjustment [Member]
Jul. 14, 2008
Other Great Plains [Member]
GMO Senior Notes [Member]
Fair value adjustment [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Senior Notes [Member]
Senior notes 8.27 percent series due 2021 [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Senior Notes [Member]
Senior notes 8.27 percent series due 2021 [Member]
Dec. 31, 2011
Other Great Plains [Member]
GMO Senior Notes [Member]
Series 7.95 percent [Member]
Dec. 31, 2010
Other Great Plains [Member]
GMO Senior Notes [Member]
Series 7.95 percent [Member]
Dec. 31, 2011
Other Great Plains [Member]
Great Plains Energy Equity Units Subordinated Notes [Member]
Dec. 31, 2010
Other Great Plains [Member]
Great Plains Energy Equity Units Subordinated Notes [Member]
May 01, 2009
Other Great Plains [Member]
Great Plains Energy Equity Units Subordinated Notes [Member]
Dec. 31, 2011
Other Great Plains [Member]
Great Plains Energy Senior Notes [Member]
Senior Notes Due 2021 [Member]
Dec. 31, 2010
Other Great Plains [Member]
Great Plains Energy Senior Notes [Member]
Senior Notes Due 2021 [Member]
Dec. 31, 2011
Other Great Plains [Member]
Great Plains Energy Senior Notes [Member]
Senor notes due 2013 [Member]
Dec. 31, 2010
Other Great Plains [Member]
Great Plains Energy Senior Notes [Member]
Senor notes due 2013 [Member]
Dec. 31, 2011
Other Great Plains [Member]
Great Plains Energy Senior Notes [Member]
Senior notes due 2017 [Member]
Dec. 31, 2010
Other Great Plains [Member]
Great Plains Energy Senior Notes [Member]
Senior notes due 2017 [Member]
Debt Instrument [Line Items]                                                                                                                                                        
Principal outstanding                         $ 350.0 [1] $ 350.0 [1] $ 250.0 [1] $ 250.0 [1] $ 0 $ 150.0 $ 250.0 [1] $ 250.0 [1] $ 400.0 $ 0 $ 2.9 $ 3.3 $ 119.3 [2],[3] $ 158.8 [2],[3] $ 400.0 [1] $ 400.0 [1] $ 50.0 $ 50.0 $ 0 [4] $ 10.0 [4] $ 73.2 $ 73.2 $ 0 [4] $ 63.3 [4] $ 23.4 $ 23.4       $ 11.2 $ 12.4 $ 7.0 $ 7.0 $ 3.0 $ 3.0 $ 6.0 $ 6.0 $ 5.6 $ 5.6 $ 7.3 [5] $ 7.3 [5] $ 5.0 [5] $ 5.0 [5] $ 500.0 $ 500.0 $ 0 $ 197.0 $ 16.3 $ 49.9     $ 80.9 $ 80.9 $ 0 $ 137.3 $ 287.5 $ 287.5   $ 350.0 [1] $ 0 [1] $ 250.0 [1] $ 250.0 [1] $ 100.0 [1] $ 100.0 [1]
Current maturities 801.4 485.7         12.7 150.3                                                             (788.7) (335.4)                                                                        
Unamortized discount             (4.2) (2.0)                                                             (0.7) (0.5)                                                                        
Long-term debt excluding current maturities 2,742.3 2,942.7         1,901.9 1,629.7                                                                                                                                        
Year due                         2018 [1]   2035 [1]       2017 [1]   2041           2019 [1]   2035 [4]   2035 [2],[3]   2035   2035 [4]   2038             2023   2023   2013   2013   2026 [5]   2028 [5]   2012               2021       2012     2021 [1]   2013 [1]   2017 [1]  
Debt Instrument Maturity Date Range Start                                             2012   2012 [2],[3]                                 2012                                                                    
Debt Instrument Maturity Date Range End                                             2018   2035                                 2021                                                                    
Interest rates (in hundredths)                         6.375% [1]   6.05% [1]   6.50%   5.85% [1]   5.30%       4.87% [2],[3]   7.15% [1]   4.65%       5.375%       4.90%         9.44%   7.17%   7.33%   7.16%   5.85%   0.164% [5]   0.353% [5]   11.875%   7.75%           8.27%   7.95%   10.00%     4.85% [1]   2.75% [1]   6.875% [1]  
Rate after amortizing gains or losses in Other Comprehensive Income (in hundredths)                         7.49% [1]   5.78% [1]       5.72% [1]               8.59% [1]                                                                                       7.34% [1]   3.67% [1]   7.33% [1]  
Bonds purchased in lieu of redemption                                                             10.0       63.3                                                                                  
Amortization of debt expense [Abstract]                                                                                                                                                        
Amortization of debt expense 8.1 6.4 4.4       3.6 2.8 2.0                                                           4.5 3.6 2.4                                                                      
Long-term debt detail [Abstract]                                                                                                                                                        
Principal outstanding covered by insurance policies                   31.0 79.5 35.9                         50.0           10.0   73.2   63.3                                                                                  
Aggregate amount of outstanding general mortgage bonds as a percentage of total capitalization, maximum (in hundredths)                   10.00% 10.00% 10.00%                                 10.00%   10.00%   10.00%   10.00%                                                                                  
Proportion of secured debt to total indebtedness, maximum (in hundredths)                   75.00% 75.00% 75.00%                                 75.00%                                                                                              
Proportion of secured debt to total indebtedness based on bond ratings, maximum (in hundredths)                   50.00% 50.00% 50.00%                                 50.00%                                                                                              
Insurance agreement covenant indebtedness to total capitalization ratio, maximum                   0.68 0.68 0.68                                 0.68                                                                                              
Additional collateral necessary due to issuance of additional mortgage bonds                                                         50.0   10.0   73.2   63.3                                                                                  
Amortization expense related to the fair value adjustment                                                                                                                       33.6 34.6 33.0                            
Number of outstanding equity units (in number of units)                                                                                                                                       5,750,000                
Payment terms                                                                                                                                       Great Plains Energy makes quarterly contract adjustment payments at the rate of 2.00% per year of the stated amount of $50 per Equity Unit and interest payments at the rate of 10.00% per year on the subordinated notes.                
Present value of the contract adjustment payments                                                                                                                                           15.1            
Compliance with Syncora bond insurance covenant                   in compliance in compliance in compliance                         in compliance                                                                                                      
Purchase accounting adjustment                                                                                                                             133.3                          
Long-term debt maturities for the next five years [Abstract]                                                                                                                                                        
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months 801.4           12.7                                                                                                                                          
Long-term Debt, Maturities, Repayments of Principal in Year Two 263.1           0.4                                                                                                                                          
Long-term Debt, Maturities, Repayments of Principal in Year Three 1.5           0.4                                                                                                                                          
Long-term Debt, Maturities, Repayments of Principal in Year Four 15.5           14.4                                                                                                                                          
Long-term Debt, Maturities, Repayments of Principal in Year Five $ 1.6           $ 0.4                                                                                                                                          
Equity Units Subordinated Notes [Abstract]                                                                                                                                                        
Settlement rate (in common shares)       2.9762 to 1 $50 divided by the applicable market value to 1 3.5714 to 1                                                                                                                                            
Market value per Equity Unit       Greater than $50 per Equity Unit [6] Equal to $50 per Equity Unit [6] Less than $50 per Equity Unit [6]                                                                                                                                            
Applicable market value, minimum       $ 16.80 $ 14.00                                                                                                                                              
Applicable market value, maximum         $ 16.80 $ 14.00                                                                                                                                            
[1] Rate after amortizing gains/losses recognized in OCI on settlements of interest rate hedging instruments
[2] Weighted-average interest rates at December 31, 2011
[3] December 31, 2011, does not include $39.5 million EIRR Series 1993B bonds because the bonds have been repurchased and are held by KCP&L
[4] December 31, 2011, does not include $63.3 million EIRR Series 2007 A-1and $10.0 million EIRR Series 2007 A-2 bonds because the bonds have been repurchased and are held by KCP&L
[5] Variable rate
[6] Assumes that the market price of the Company's common stock on June 15, 2012, is the same as the applicable market value.