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EMPLOYEE BENEFIT PLANS - Pension and post-retirement plan assets by asset category (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | $ 160,523 | $ 147,145 |
Defined benefit plan, fair value of plan assets, subtotal excluding accrued income and plan receivables | 160,238 | 142,179 |
Fair Value, Inputs, Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets, subtotal excluding accrued income and plan receivables | 121,655 | 105,929 |
Fair Value, Inputs, Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets, subtotal excluding accrued income and plan receivables | 27,297 | 26,888 |
Fair Value, Inputs, Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 11,286 | 9,362 | $ 8,776
Defined benefit plan, fair value of plan assets, subtotal excluding accrued income and plan receivables | 11,286 | 9,362 |
Cash and equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 1,867 | 600 |
Cash and equivalents | Fair Value, Inputs, Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 1,867 | 600 |
Cash and equivalents | Fair Value, Inputs, Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Cash and equivalents | Fair Value, Inputs, Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Government agency securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 36,825 | 39,811 |
Government agency securities | Fair Value, Inputs, Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 32,217 | 33,675 |
Government agency securities | Fair Value, Inputs, Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 4,608 | 6,136 |
Government agency securities | Fair Value, Inputs, Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Debt instruments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 3,769 | 2,901 |
Debt instruments | Fair Value, Inputs, Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 1,063 | 179 |
Debt instruments | Fair Value, Inputs, Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 2,706 | 2,722 |
Debt instruments | Fair Value, Inputs, Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 84,129 | 68,987 |
Equity securities | Fair Value, Inputs, Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 84,129 | 68,987 |
Equity securities | Fair Value, Inputs, Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Equity securities | Fair Value, Inputs, Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Commingled funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 11,286 | 9,362 |
Commingled funds | Fair Value, Inputs, Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Commingled funds | Fair Value, Inputs, Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Commingled funds | Fair Value, Inputs, Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 11,286 | 9,362 |
Limited partnerships and hedge fund investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 19,823 | 17,867 |
Limited partnerships and hedge fund investments | Fair Value, Inputs, Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Limited partnerships and hedge fund investments | Fair Value, Inputs, Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 19,823 | 17,867 |
Limited partnerships and hedge fund investments | Fair Value, Inputs, Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Other Securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 2,539 | 2,651 |
Other Securities | Fair Value, Inputs, Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 2,379 | 2,488 |
Other Securities | Fair Value, Inputs, Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 160 | 163 |
Other Securities | Fair Value, Inputs, Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | 0 | 0 |
Accrued income and plan receivables |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, fair value of plan assets | $ 285 | $ 4,966 |

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