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Fair Value Measurements Fair Value Long-Term Debt (Details) (USD $) In Billions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair value of financial instruments [Abstract]
Long-term Debt	$ 3.5	$ 3.5
Carrying (Reported) Amount, Fair Value Disclosure [Member] \| Kansas City Power and Light Company [Member]
Fair value of financial instruments [Abstract]
Long-term Debt	2.3	2.3
Fair Value, Inputs, Level 2 [Member] \| Estimate of Fair Value Measurement [Member]
Fair value of financial instruments [Abstract]
Long-term debt fair value	3.9	3.7
Fair Value, Inputs, Level 2 [Member] \| Estimate of Fair Value Measurement [Member] \| Kansas City Power and Light Company [Member]
Fair value of financial instruments [Abstract]
Long-term debt fair value	$ 2.6	$ 2.5

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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