EMPLOYEE BENEFIT PLANS - FAIR VALUE (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	$ 288,634	$ 279,410	$ 271,743
OPEB
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	83,926	77,980	$ 71,534
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	276,014	277,993
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Money market funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	0	8,207
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Large Cap Index
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	89,374	97,016
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Extended Market Index
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	16,548	17,741
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | International Stock
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	49,929	53,516
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	15,794	11,754
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Core Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Opportunistic Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Ultra Short Duration
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | High Yield Bond Fund
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	24,328	25,720
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension | Long Duration Fund
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	80,041	64,039
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	80,484	77,599
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Money market funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	21	2,273
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Large Cap Index
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	25,474	27,340
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Extended Market Index
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	5,036	5,014
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | International Stock
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	14,564	14,874
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	4,764	3,264
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Core Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	10,570	7,970
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Opportunistic Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	6,365	4,798
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Ultra Short Duration
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	6,340	4,830
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | High Yield Bond Fund
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	7,350	7,236
Quoted Prices in Active Markets for Identical Assets (Level 1) | OPEB | Long Duration Fund
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets		0
Fair Value, Inputs, Level 1, 2 and 3 | Pension
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	276,014	277,993
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Money market funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	0	8,207
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Large Cap Index
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	89,374	97,016
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Extended Market Index
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	16,548	17,741
Fair Value, Inputs, Level 1, 2 and 3 | Pension | International Stock
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	49,929	53,516
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	15,794	11,754
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Core Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Opportunistic Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Ultra Short Duration
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Fair Value, Inputs, Level 1, 2 and 3 | Pension | High Yield Bond Fund
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	24,328	25,720
Fair Value, Inputs, Level 1, 2 and 3 | Pension | Long Duration Fund
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	80,041	64,039
Fair Value, Inputs, Level 1, 2 and 3 | OPEB
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	80,484	77,599
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Money market funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	21	2,273
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Large Cap Index
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	25,474	27,340
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Extended Market Index
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	5,036	5,014
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | International Stock
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	14,564	14,874
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	4,764	3,264
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Core Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	10,570	7,970
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Opportunistic Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	6,365	4,798
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Ultra Short Duration
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	6,340	4,830
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | High Yield Bond Fund
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	7,350	7,236
Fair Value, Inputs, Level 1, 2 and 3 | OPEB | Long Duration Fund
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	0	0
Investments measured at net asset value | Pension | Common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	12,620	1,417
Investments measured at net asset value | OPEB | Common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets	$ 3,442	$ 381