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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income (Expense) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Other Income Expense Net [Line Items] |  |  |
Non-service cost component of net benefit cost | $ (58.6) | $ (55.6) | $ (47.8)
Other | (19.6) | (21.3) | (30.9)
Other expense | (78.2) | (76.9) | (78.7)
Evergy Kansas Central |  |  |
Other Income Expense Net [Line Items] |  |  |
Non-service cost component of net benefit cost | (21.2) | (20.1) | (23.5)
Other | (17.7) | (20.0) | (23.3)
Other expense | (38.9) | (40.1) | (46.8)
Evergy Metro |  |  |
Other Income Expense Net [Line Items] |  |  |
Non-service cost component of net benefit cost | (24.2) | (20.9) | (25.9)
Other | (1.3) | (0.5) | (5.0)
Other expense | $ (25.5) | $ (21.4) | $ (30.9)

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