Plant to be Retired, Net (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Plant to be Retired, Net [Line Items]
Plant to be Retired, Net	$ 149.2	$ 0.0

[Line Items] for Schedule of Plant to be Retired, Net [Table]

No definition available.

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Namespace Prefix:	gxp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration