Guarantees (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Guarantor Obligations [Line Items]
Origin and purpose	In the ordinary course of business, Evergy and certain of its subsidiaries enter into various agreements providing financial or performance assurance to third parties on behalf of certain subsidiaries. Such agreements include, for example, guarantees and letters of credit. These agreements are entered into primarily to support or enhance the creditworthiness otherwise attributed to a subsidiary on a stand-alone basis, thereby facilitating the extension of sufficient credit to accomplish the subsidiary's intended business purposes.
Maximum exposure	$ 111.3
Direct Guarantee [Member]
Guarantor Obligations [Line Items]
Origin and purpose	direct guarantees to GMO counterparties
Maximum exposure	$ 17.0
Obligations term	expire in 2020
Guarantee of Indebtedness of Others [Member] | Long-term Debt [Member]
Guarantor Obligations [Line Items]
Origin and purpose	guarantee of GMO long-term debt
Maximum exposure	$ 94.3
Obligations term	maturity dates ranging from 2019 to 2023
Guarantee of Indebtedness of Others [Member] | Commercial Paper [Member]
Guarantor Obligations [Line Items]
Origin and purpose	guaranteed GMO's commercial paper program
Maximum exposure	$ 140.4