Short-term Borrowings and Short-term Bank Lines of Credit (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 1,800.0
Line of Credit Facility, Current Borrowing Capacity	2,500.0
Commercial Paper [Member]
Short-term Debt [Line Items]
Long-term Line of Credit	675.9
Standby Letters of Credit [Member]
Short-term Debt [Line Items]
Amount of letters of credit outstanding	24.1
Master Credit Facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	2,500.0
Long-term Line of Credit	$ 0.0
Credit facility expiration date	Dec. 31, 2023
Evergy [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 449.0	$ 691.8
Line of Credit Facility, Interest Rate at Period End	0.00%	1.83%
Line of Credit Facility, Current Borrowing Capacity	$ 450.0	$ 979.3
Evergy [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Long-term Line of Credit		275.7
Evergy [Member] | Standby Letters of Credit [Member]
Short-term Debt [Line Items]
Amount of letters of credit outstanding	1.0	11.8
Evergy [Member] | Master Credit Facility [Member]
Short-term Debt [Line Items]
Debt Instrument, Debt Default, Threshold	100.0
Long-term Line of Credit	$ 0.0	0.0
Line of Credit Facility, Covenant Terms	A default by any borrower under the facility or one of their significant subsidiaries on other indebtedness totaling more than $100.0 million constitutes a default under the facility. Under the terms of this facility, each of Evergy, Westar Energy, KCP&L and GMO is required to maintain a total indebtedness to total capitalization ratio, as defined in the facility, of not greater than 0.65 to 1.00 at all times.
Line of Credit Facility, Covenant Compliance	in compliance
Evergy [Member] | Master Credit Facility [Member] | Maximum [Member]
Short-term Debt [Line Items]
Debt Instrument, debt to capitalization ratio	65.00%
KCPL [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 388.1	$ 429.8
Line of Credit Facility, Interest Rate at Period End	2.50%	1.95%
Line of Credit Facility, Current Borrowing Capacity	$ 600.0	$ 600.0
KCPL [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Long-term Line of Credit	209.2	167.5
KCPL [Member] | Standby Letters of Credit [Member]
Short-term Debt [Line Items]
Amount of letters of credit outstanding	2.7	2.7
KCPL [Member] | Master Credit Facility [Member]
Short-term Debt [Line Items]
Debt Instrument, Debt Default, Threshold	100.0
Long-term Line of Credit	$ 0.0	0.0
Line of Credit Facility, Covenant Terms	A default by any borrower under the facility or one of their significant subsidiaries on other indebtedness totaling more than $100.0 million constitutes a default under the facility. Under the terms of this facility, each of Evergy, Westar Energy, KCP&L and GMO is required to maintain a total indebtedness to total capitalization ratio, as defined in the facility, of not greater than 0.65 to 1.00 at all times.
Line of Credit Facility, Covenant Compliance	in compliance
KCPL [Member] | Master Credit Facility [Member] | Maximum [Member]
Short-term Debt [Line Items]
Debt Instrument, debt to capitalization ratio	65.00%
Westar Energy Inc [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 655.4	$ 691.8
Line of Credit Facility, Interest Rate at Period End	2.42%	1.83%
Line of Credit Facility, Current Borrowing Capacity	$ 1,000.0	$ 979.3
Westar Energy Inc [Member] | Commercial Paper [Member]
Short-term Debt [Line Items]
Long-term Line of Credit	326.3	275.7
Westar Energy Inc [Member] | Standby Letters of Credit [Member]
Short-term Debt [Line Items]
Amount of letters of credit outstanding	18.3	11.8
Westar Energy Inc [Member] | Master Credit Facility [Member]
Short-term Debt [Line Items]
Debt Instrument, Debt Default, Threshold	100.0
Long-term Line of Credit	$ 0.0	0.0
Line of Credit Facility, Covenant Terms	A default by any borrower under the facility or one of their significant subsidiaries on other indebtedness totaling more than $100.0 million constitutes a default under the facility. Under the terms of this facility, each of Evergy, Westar Energy, KCP&L and GMO is required to maintain a total indebtedness to total capitalization ratio, as defined in the facility, of not greater than 0.65 to 1.00 at all times.
Line of Credit Facility, Covenant Compliance	in compliance
Westar Energy Inc [Member] | Master Credit Facility [Member] | Maximum [Member]
Short-term Debt [Line Items]
Debt Instrument, debt to capitalization ratio	65.00%
Kcpl Greater Missouri Operations Member
Short-term Debt [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 307.5
Line of Credit Facility, Interest Rate at Period End	2.46%
Line of Credit Facility, Current Borrowing Capacity	$ 450.0
Kcpl Greater Missouri Operations Member | Commercial Paper [Member]
Short-term Debt [Line Items]
Long-term Line of Credit	140.4
Kcpl Greater Missouri Operations Member | Standby Letters of Credit [Member]
Short-term Debt [Line Items]
Amount of letters of credit outstanding	2.1
Kcpl Greater Missouri Operations Member | Master Credit Facility [Member]
Short-term Debt [Line Items]
Debt Instrument, Debt Default, Threshold	100.0
Long-term Line of Credit	$ 0.0
Line of Credit Facility, Covenant Terms	A default by any borrower under the facility or one of their significant subsidiaries on other indebtedness totaling more than $100.0 million constitutes a default under the facility. Under the terms of this facility, each of Evergy, Westar Energy, KCP&L and GMO is required to maintain a total indebtedness to total capitalization ratio, as defined in the facility, of not greater than 0.65 to 1.00 at all times.
Line of Credit Facility, Covenant Compliance	in compliance
Kcpl Greater Missouri Operations Member | Master Credit Facility [Member] | Maximum [Member]
Short-term Debt [Line Items]
Debt Instrument, debt to capitalization ratio	65.00%
Wester Energy Revolving Credit Facility One [Member] | Westar Energy Inc [Member] | Master Credit Facility [Member]
Short-term Debt [Line Items]
Line Of Credit, Expired		20.7
Line of Credit Facility, Current Borrowing Capacity		730.0
Wester Energy Revolving Credit Facility Two [Member] | Westar Energy Inc [Member] | Master Credit Facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 270.0