FAIR VALUE MEASUREMENTS - Recurring (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets
Total nuclear decommissioning trust	$ 518.3	$ 472.1
Total rabbi trust	44.6	43.8
Total self-insured health plan trust	12.6	12.4
Total Assets	575.5	528.3
Liabilities
Total	19.2	5.4
Interest rate swaps
Liabilities
Interest rate swaps	19.2	5.4
Level 1
Assets
Total nuclear decommissioning trust	443.8	396.0
Total rabbi trust	0.6	0.2
Total self-insured health plan trust	7.6	8.8
Total Assets	452.0	405.0
Liabilities
Total	0.0	0.0
Level 1 | Interest rate swaps
Liabilities
Interest rate swaps	0.0	0.0
Level 2
Assets
Total nuclear decommissioning trust	32.7	33.5
Total rabbi trust	0.0	0.0
Total self-insured health plan trust	5.0	3.6
Total Assets	37.7	37.1
Liabilities
Total	19.2	5.4
Level 2 | Interest rate swaps
Liabilities
Interest rate swaps	19.2	5.4
Level 3
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Total self-insured health plan trust	0.0	0.0
Total Assets	0.0	0.0
Liabilities
Total	0.0	0.0
Level 3 | Interest rate swaps
Liabilities
Interest rate swaps	0.0	0.0
NAV
Assets
Total nuclear decommissioning trust	41.8	42.6
Total rabbi trust	44.0	43.6
Total self-insured health plan trust	0.0	0.0
Total Assets	85.8	86.2
Liabilities
Total	0.0	0.0
NAV | Interest rate swaps
Liabilities
Interest rate swaps	0.0	0.0
Westar Energy
Assets
Total nuclear decommissioning trust	248.4	227.5
Total rabbi trust	31.1	30.6
Total Assets	279.5	258.1
Westar Energy | Core bond fund
Assets
Total nuclear decommissioning trust	34.6	37.5
Total rabbi trust	24.7	24.8
Westar Energy | High-yield bond fund
Assets
Total nuclear decommissioning trust	20.6	18.9
Westar Energy | Emerging markets bond fund
Assets
Total nuclear decommissioning trust	16.4	15.4
Westar Energy | Combination debt/equity/other fund
Assets
Total nuclear decommissioning trust	14.7	12.9
Total rabbi trust	6.2	5.6
Westar Energy | Alternative investments fund
Assets
Total nuclear decommissioning trust	23.3	24.1
Westar Energy | Real estate securities fund
Assets
Total nuclear decommissioning trust	12.0	11.8
Westar Energy | Cash equivalents
Assets
Total nuclear decommissioning trust	0.2	0.1
Total rabbi trust	0.2	0.2
Westar Energy | Level 1
Assets
Total nuclear decommissioning trust	206.6	184.9
Total rabbi trust	0.2	0.2
Total Assets	206.8	185.1
Westar Energy | Level 1 | Core bond fund
Assets
Total nuclear decommissioning trust	34.6	37.5
Total rabbi trust	0.0	0.0
Westar Energy | Level 1 | High-yield bond fund
Assets
Total nuclear decommissioning trust	20.6	18.9
Westar Energy | Level 1 | Emerging markets bond fund
Assets
Total nuclear decommissioning trust	16.4	15.4
Westar Energy | Level 1 | Combination debt/equity/other fund
Assets
Total nuclear decommissioning trust	14.7	12.9
Total rabbi trust	0.0	0.0
Westar Energy | Level 1 | Alternative investments fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 1 | Real estate securities fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 1 | Cash equivalents
Assets
Total nuclear decommissioning trust	0.2	0.1
Total rabbi trust	0.2	0.2
Westar Energy | Level 2
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Total Assets	0.0	0.0
Westar Energy | Level 2 | Core bond fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Westar Energy | Level 2 | High-yield bond fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 2 | Emerging markets bond fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 2 | Combination debt/equity/other fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Westar Energy | Level 2 | Alternative investments fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 2 | Real estate securities fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 2 | Cash equivalents
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Westar Energy | Level 3
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Total Assets	0.0	0.0
Westar Energy | Level 3 | Core bond fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Westar Energy | Level 3 | High-yield bond fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 3 | Emerging markets bond fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 3 | Combination debt/equity/other fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Westar Energy | Level 3 | Alternative investments fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 3 | Real estate securities fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Level 3 | Cash equivalents
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Westar Energy | NAV
Assets
Total nuclear decommissioning trust	41.8	42.6
Total rabbi trust	30.9	30.4
Total Assets	72.7	73.0
Westar Energy | NAV | Core bond fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	24.7	24.8
Westar Energy | NAV | High-yield bond fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | NAV | Emerging markets bond fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | NAV | Combination debt/equity/other fund
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	6.2	5.6
Westar Energy | NAV | Alternative investments fund
Assets
Total nuclear decommissioning trust	23.3	24.1
Westar Energy | NAV | Real estate securities fund
Assets
Total nuclear decommissioning trust	12.0	11.8
Westar Energy | NAV | Cash equivalents
Assets
Total nuclear decommissioning trust	0.0	0.0
Total rabbi trust	0.0	0.0
Westar Energy | Domestic | Equity securities
Assets
Total nuclear decommissioning trust	80.1	70.6
Westar Energy | Domestic | Level 1 | Equity securities
Assets
Total nuclear decommissioning trust	73.6	63.9
Westar Energy | Domestic | Level 2 | Equity securities
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Domestic | Level 3 | Equity securities
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | Domestic | NAV | Equity securities
Assets
Total nuclear decommissioning trust	6.5	6.7
Westar Energy | International | Equity securities
Assets
Total nuclear decommissioning trust	46.5	36.2
Westar Energy | International | Level 1 | Equity securities
Assets
Total nuclear decommissioning trust	46.5	36.2
Westar Energy | International | Level 2 | Equity securities
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | International | Level 3 | Equity securities
Assets
Total nuclear decommissioning trust	0.0	0.0
Westar Energy | International | NAV | Equity securities
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L
Assets
Total nuclear decommissioning trust	269.9	244.6
Total self-insured health plan trust	12.6	12.4
Total Assets	282.5	257.0
KCP&L | Equity securities
Assets
Total nuclear decommissioning trust	188.3	166.6
Total self-insured health plan trust	0.5	0.5
KCP&L | Cash equivalents
Assets
Total nuclear decommissioning trust	2.9	1.7
KCP&L | U.S. Treasury
Assets
Total nuclear decommissioning trust	46.0	42.1
KCP&L | U.S. Agency
Assets
Total nuclear decommissioning trust	0.4	0.4
KCP&L | State and local obligations
Assets
Total nuclear decommissioning trust	2.1	2.1
KCP&L | Corporate bonds
Assets
Total nuclear decommissioning trust	30.6	30.9
KCP&L | Foreign governments
Assets
Total nuclear decommissioning trust	0.1	0.1
KCP&L | Other
Assets
Total nuclear decommissioning trust	(0.5)	0.7
KCP&L | Debt securities
Assets
Total self-insured health plan trust	4.3	3.9
KCP&L | Cash and cash equivalents
Assets
Total self-insured health plan trust	6.7	8.0
KCP&L | Other
Assets
Total self-insured health plan trust	1.1
KCP&L | Level 1
Assets
Total nuclear decommissioning trust	237.2	211.1
Total self-insured health plan trust	7.6	8.8
Total Assets	244.8	219.9
KCP&L | Level 1 | Equity securities
Assets
Total nuclear decommissioning trust	188.3	166.6
Total self-insured health plan trust	0.5	0.5
KCP&L | Level 1 | Cash equivalents
Assets
Total nuclear decommissioning trust	2.9	1.7
KCP&L | Level 1 | U.S. Treasury
Assets
Total nuclear decommissioning trust	46.0	42.1
KCP&L | Level 1 | U.S. Agency
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 1 | State and local obligations
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 1 | Corporate bonds
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 1 | Foreign governments
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 1 | Other
Assets
Total nuclear decommissioning trust	0.0	0.7
KCP&L | Level 1 | Debt securities
Assets
Total self-insured health plan trust	0.4	0.3
KCP&L | Level 1 | Cash and cash equivalents
Assets
Total self-insured health plan trust	6.7	8.0
KCP&L | Level 1 | Other
Assets
Total self-insured health plan trust	0.0
KCP&L | Level 2
Assets
Total nuclear decommissioning trust	32.7	33.5
Total self-insured health plan trust	5.0	3.6
Total Assets	37.7	37.1
KCP&L | Level 2 | Equity securities
Assets
Total nuclear decommissioning trust	0.0	0.0
Total self-insured health plan trust	0.0	0.0
KCP&L | Level 2 | Cash equivalents
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 2 | U.S. Treasury
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 2 | U.S. Agency
Assets
Total nuclear decommissioning trust	0.4	0.4
KCP&L | Level 2 | State and local obligations
Assets
Total nuclear decommissioning trust	2.1	2.1
KCP&L | Level 2 | Corporate bonds
Assets
Total nuclear decommissioning trust	30.6	30.9
KCP&L | Level 2 | Foreign governments
Assets
Total nuclear decommissioning trust	0.1	0.1
KCP&L | Level 2 | Other
Assets
Total nuclear decommissioning trust	(0.5)	0.0
KCP&L | Level 2 | Debt securities
Assets
Total self-insured health plan trust	3.9	3.6
KCP&L | Level 2 | Cash and cash equivalents
Assets
Total self-insured health plan trust	0.0	0.0
KCP&L | Level 2 | Other
Assets
Total self-insured health plan trust	1.1
KCP&L | Level 3
Assets
Total nuclear decommissioning trust	0.0	0.0
Total self-insured health plan trust	0.0	0.0
Total Assets	0.0	0.0
KCP&L | Level 3 | Equity securities
Assets
Total nuclear decommissioning trust	0.0	0.0
Total self-insured health plan trust	0.0	0.0
KCP&L | Level 3 | Cash equivalents
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 3 | U.S. Treasury
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 3 | U.S. Agency
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 3 | State and local obligations
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 3 | Corporate bonds
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 3 | Foreign governments
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 3 | Other
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | Level 3 | Debt securities
Assets
Total self-insured health plan trust	0.0	0.0
KCP&L | Level 3 | Cash and cash equivalents
Assets
Total self-insured health plan trust	0.0	0.0
KCP&L | Level 3 | Other
Assets
Total self-insured health plan trust	0.0
KCP&L | NAV
Assets
Total nuclear decommissioning trust	0.0	0.0
Total self-insured health plan trust	0.0	0.0
Total Assets	0.0	0.0
KCP&L | NAV | Equity securities
Assets
Total nuclear decommissioning trust	0.0	0.0
Total self-insured health plan trust	0.0	0.0
KCP&L | NAV | Cash equivalents
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | NAV | U.S. Treasury
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | NAV | U.S. Agency
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | NAV | State and local obligations
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | NAV | Corporate bonds
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | NAV | Foreign governments
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | NAV | Other
Assets
Total nuclear decommissioning trust	0.0	0.0
KCP&L | NAV | Debt securities
Assets
Total self-insured health plan trust	0.0	0.0
KCP&L | NAV | Cash and cash equivalents
Assets
Total self-insured health plan trust	0.0	0.0
KCP&L | NAV | Other
Assets
Total self-insured health plan trust	0.0
Other Evergy
Assets
Total rabbi trust	13.5	13.2
Liabilities
Total	19.2	5.4
Other Evergy | Interest rate swaps
Liabilities
Interest rate swaps	19.2	5.4
Other Evergy | Cash and cash equivalents
Assets
Total rabbi trust	0.4
Other Evergy | Fixed income fund
Assets
Total rabbi trust	13.1	13.2
Other Evergy | Level 1
Assets
Total rabbi trust	0.4	0.0
Liabilities
Total	0.0	0.0
Other Evergy | Level 1 | Interest rate swaps
Liabilities
Interest rate swaps	0.0	0.0
Other Evergy | Level 1 | Cash and cash equivalents
Assets
Total rabbi trust	0.4
Other Evergy | Level 1 | Fixed income fund
Assets
Total rabbi trust	0.0	0.0
Other Evergy | Level 2
Assets
Total rabbi trust	0.0	0.0
Liabilities
Total	19.2	5.4
Other Evergy | Level 2 | Interest rate swaps
Liabilities
Interest rate swaps	19.2	5.4
Other Evergy | Level 2 | Cash and cash equivalents
Assets
Total rabbi trust	0.0
Other Evergy | Level 2 | Fixed income fund
Assets
Total rabbi trust	0.0	0.0
Other Evergy | Level 3
Assets
Total rabbi trust	0.0	0.0
Liabilities
Total	0.0	0.0
Other Evergy | Level 3 | Interest rate swaps
Liabilities
Interest rate swaps	0.0	0.0
Other Evergy | Level 3 | Cash and cash equivalents
Assets
Total rabbi trust	0.0
Other Evergy | Level 3 | Fixed income fund
Assets
Total rabbi trust	0.0	0.0
Other Evergy | NAV
Assets
Total rabbi trust	13.1	13.2
Liabilities
Total	0.0	0.0
Other Evergy | NAV | Interest rate swaps
Liabilities
Interest rate swaps	0.0	0.0
Other Evergy | NAV | Cash and cash equivalents
Assets
Total rabbi trust	0.0
Other Evergy | NAV | Fixed income fund
Assets
Total rabbi trust	$ 13.1	$ 13.2