Taxes Deferred Income Taxes (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2013
Net Operating Losses Reclassified	$ 60.3

Amount of Net Operating Losses reclassified from current deferred income tax asset to noncurrent deferred income tax liability driven by the expected timing of their utilization due to the extension of 50% bonus depreciation with the January 2, 2013, enactment of the American Taxpayer Relief Act of 2012.

No definition available.

Name:	gxp_NetOperatingLossesReclassified
Namespace Prefix:	gxp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration