Legal Proceedings Loss Contingencies (Details) (Pending Litigation [Member], KCPL Greater Missouri Operations [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 2013
Pending Litigation [Member] \| KCPL Greater Missouri Operations [Member]
Loss Contingencies [Line Items]
Approximate amount of refunds received as a net purchaser of power through settlements with sellers of power	$ 8
Estimated additional refunds entitled to receive	$ 12

The approximate amount of refunds received by MPS Merchant Services, Inc., a wholly owned subsidiary of KCP and L Greater Missouri Operations Company, for net purchases of power in the California energy markets between October 2, 2000 through June 20, 2001.

No definition available.

Name:	gxp_ApproximateAmountOfRefundReceivedFromVariousProviders
Namespace Prefix:	gxp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant