Summary of Significant Accounting Policies Summary of Significant Accounting Policies Utility Plant (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Utility Plant, at original cost
Public Utilities, Property, Plant and Equipment, Plant in Service	$ 11,387.8	$ 11,160.5
Kansas City Power and Light Company [Member]
Utility Plant, at original cost
Public Utilities, Property, Plant and Equipment, Plant in Service	$ 8,118.6	$ 7,971.4