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PENSION PLANS AND POST-RETIREMENT BENEFITS (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Pension Benefits |  |
Components of net periodic benefit costs [Abstract] |  |
Service cost | $ 19.1 | $ 8.0
Interest cost | 27.5 | 12.7
Expected return on plan assets | (27.1) | (14.0)
Prior service cost | 0.5 | 0.2
Recognized net actuarial (gain)/loss | 6.9 | 8.2
Net periodic benefit costs before regulatory adjustment and intercompany allocations | 26.9 | 15.1
Regulatory adjustment | (0.1) | 2.8
Intercompany allocations | 0.0 | 0.0
Net periodic benefit costs | 26.8 | 17.9
Contributions by employer | 12.3 |
Contributions by employer, remainder of fiscal year | 103.2 |
Pension Benefits | Westar Energy |  |
Components of net periodic benefit costs [Abstract] |  |
Service cost | 7.3 | 8.0
Interest cost | 13.4 | 12.7
Expected return on plan assets | (13.7) | (14.0)
Prior service cost | 0.4 | 0.2
Recognized net actuarial (gain)/loss | 6.4 | 8.2
Net periodic benefit costs before regulatory adjustment and intercompany allocations | 13.8 | 15.1
Regulatory adjustment | 0.5 | 2.8
Intercompany allocations | 0.0 | 0.0
Net periodic benefit costs | 14.3 | 17.9
Contributions by employer | 7.1 |
Contributions by employer, remainder of fiscal year | 29.9 |
Pension Benefits | KCP&L |  |
Components of net periodic benefit costs [Abstract] |  |
Service cost | 11.8 | 12.2
Interest cost | 14.1 | 12.7
Expected return on plan assets | (12.3) | (13.9)
Prior service cost | 0.2 | 0.2
Recognized net actuarial (gain)/loss | 12.2 | 11.4
Net periodic benefit costs before regulatory adjustment and intercompany allocations | 26.0 | 22.6
Regulatory adjustment | (0.6) | 0.4
Intercompany allocations | (6.9) | (5.5)
Net periodic benefit costs | 18.5 | 17.5
Contributions by employer | 5.2 |
Contributions by employer, remainder of fiscal year | 73.3 |
Post-Retirement Benefits |  |
Components of net periodic benefit costs [Abstract] |  |
Service cost | 0.6 | 0.3
Interest cost | 2.6 | 1.2
Expected return on plan assets | (2.4) | (1.7)
Prior service cost | 0.1 | 0.1
Recognized net actuarial (gain)/loss | (0.3) | (0.1)
Net periodic benefit costs before regulatory adjustment and intercompany allocations | 0.6 | (0.2)
Regulatory adjustment | (0.7) | (0.4)
Intercompany allocations | 0.0 | 0.0
Net periodic benefit costs | (0.1) | (0.6)
Contributions by employer | 2.8 |
Post-Retirement Benefits | Westar Energy |  |
Components of net periodic benefit costs [Abstract] |  |
Service cost | 0.3 | 0.3
Interest cost | 1.4 | 1.2
Expected return on plan assets | (1.7) | (1.7)
Prior service cost | 0.1 | 0.1
Recognized net actuarial (gain)/loss | (0.1) | (0.1)
Net periodic benefit costs before regulatory adjustment and intercompany allocations | 0.0 | (0.2)
Regulatory adjustment | (0.8) | (0.4)
Intercompany allocations | 0.0 | 0.0
Net periodic benefit costs | (0.8) | (0.6)
Contributions by employer | 0.7 |
Post-Retirement Benefits | KCP&L |  |
Components of net periodic benefit costs [Abstract] |  |
Service cost | 0.3 | 0.5
Interest cost | 1.2 | 1.2
Expected return on plan assets | (0.7) | (0.7)
Prior service cost | 0.0 | 0.0
Recognized net actuarial (gain)/loss | (0.4) | 0.0
Net periodic benefit costs before regulatory adjustment and intercompany allocations | 0.4 | 1.0
Regulatory adjustment | 0.1 | (0.1)
Intercompany allocations | (0.1) | (0.3)
Net periodic benefit costs | 0.4 | $ 0.6
Contributions by employer | $ 2.1 |
Wolf Creek |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Jointly owned utility plant, proportionate ownership share | 94.00% |
Wolf Creek | Westar Energy |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Jointly owned utility plant, proportionate ownership share | 47.00% |
Wolf Creek | KCP&L |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Jointly owned utility plant, proportionate ownership share | 47.00% |

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