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FAIR VALUE MEASUREMENTS - Gains (Losses) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | $ 45.2 | $ 82.2 | $ (53.6)
Nuclear decommissioning trust - equity securities |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | 45.5 | 74.0 | (54.1)
Nuclear decommissioning trust - debt securities |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | 5.3 | 5.1 | (0.5)
Rabbi trusts - equity securities |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | (5.6) | 3.1 | 1.0
Evergy Kansas Central |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | 15.8 | 36.5 | (30.8)
Evergy Kansas Central | Nuclear decommissioning trust - equity securities |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | 21.9 | 33.3 | (31.8)
Evergy Kansas Central | Rabbi trusts - equity securities |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | (6.1) | 3.2 | 1.0
Evergy Metro |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | 28.9 | 45.8 | (23.2)
Evergy Metro | Nuclear decommissioning trust - equity securities |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | 23.6 | 40.7 | (20.7)
Evergy Metro | Nuclear decommissioning trust - debt securities |  |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |  |
Unrealized gain (losses) | $ 5.3 | $ 5.1 | $ (2.5)

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- Definition Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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