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FAIR VALUE MEASUREMENTS - Debt (Details) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Notional contract amount |  |  | $ 500,000,000.0
Fair value adjustments | $ (19,200,000) |  | (5,400,000)
Loss on derivative hedging instruments | 13,800,000 | $ 0 |
Book Value | Fair Value, Measurements, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 7,827,000,000 |  | 7,341,700,000
Book Value | Fair Value, Measurements, Recurring | Variable Interest Entities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 51,100,000 |  | 81,400,000
Fair Value | Fair Value, Measurements, Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 8,130,500,000 |  | 7,412,100,000
Fair Value | Fair Value, Measurements, Recurring | Level 2 | Variable Interest Entities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 50,900,000 |  | 81,300,000
Westar Energy | Book Value | Fair Value, Measurements, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 3,690,000,000 |  | 3,689,800,000
Westar Energy | Book Value | Fair Value, Measurements, Recurring | Variable Interest Entities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 51,100,000 |  | 81,400,000
Westar Energy | Fair Value | Fair Value, Measurements, Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 3,884,400,000 |  | 3,771,300,000
Westar Energy | Fair Value | Fair Value, Measurements, Recurring | Level 2 | Variable Interest Entities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 50,900,000 |  | 81,300,000
KCP&L | Book Value | Fair Value, Measurements, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 2,924,600,000 |  | 2,530,100,000
KCP&L | Fair Value | Fair Value, Measurements, Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt | 3,130,200,000 |  | 2,637,500,000
Long-Term debt fair value adjustment |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt gross | $ 136,800,000 |  | $ 144,800,000

X
- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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