Callaway Energy Center (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended		6 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Jun. 30, 2013 Nuclear Plant [Member] mill	Dec. 31, 2012 Nuclear Plant [Member]
Nuclear Waste Matters [Line Items]
Number of mills charged for NWF fee			1
Settlement payment			$ 6
Assumed life of plant, in years			40 years
Annual decommissioning costs included in costs of service	$ 7	$ 7		$ 7
Missouri Public Service Commission, Requirement to file updated cost study and funding analysis for decommissioning energy center, Period			3 years