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FAIR VALUE MEASUREMENTS - Gain (Loss) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | $ 40.5 | $ (12.2)
Nuclear decommissioning trust - equity securities |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | 37.9 | (11.7)
Rabbi trusts |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | 0.5 | (0.5)
Nuclear decommissioning trust - debt securities |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | 2.1 | 0.0
Westar Energy |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | 18.5 | (12.2)
Westar Energy | Nuclear decommissioning trust - equity securities |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | 17.2 | (11.7)
Westar Energy | Rabbi trusts |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | 1.3 | (0.5)
KCP&L |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | 22.8 | (5.2)
KCP&L | Nuclear decommissioning trust - equity securities |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | 20.7 | (3.6)
KCP&L | Nuclear decommissioning trust - debt securities |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gain (losses) | $ 2.1 | $ (1.6)

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- Definition Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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