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EMPLOYEE BENEFIT PLANS - SUMMARY OF CHANGE IN FUNDED STATUS AND LIABILITIES (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Mar. 31, 2020 | Mar. 31, 2019 | Sep. 30, 2019
Change in plan assets |  |  |  |  |
Employer contributions |  |  | $ 0 | $ 0 |
Postemployment employee (liability) |  |  |  |  |
Noncurrent | $ (195,441,000) |  | (195,441,000) |  | $ (246,517,000)
Pension |  |  |  |  |
Change in Benefit Obligation |  |  |  |  |
Service cost | 2,055,000 | $ 1,846,000 | 4,111,000 | 3,691,000 |
Interest cost | 2,646,000 | 3,043,000 | 5,293,000 | 6,086,000 |
OPEB |  |  |  |  |
Change in Benefit Obligation |  |  |  |  |
Benefit obligation at beginning of period |  |  | 260,003,000 | 196,785,000 | 196,785,000
Service cost | 1,072,000 | 1,101,000 | 2,427,000 | 2,202,000 | 4,404,000
Interest cost | 1,509,000 | $ 2,081,000 | 3,513,000 | 4,162,000 | 8,324,000
Plan participants’ contributions |  |  |  |  | 210,000
Actuarial loss (gain) |  |  |  |  | 54,700,000
Benefits paid, net of retiree subsidies received |  |  |  |  | (4,420,000)
Benefit obligation at end of period |  |  |  |  | 260,003,000
Change in plan assets |  |  |  |  |
Fair value of plan assets at beginning of period |  |  | 83,926,000 | $ 77,980,000 | 77,980,000
Actual return on plan assets |  |  |  |  | 2,499,000
Employer contributions |  |  |  |  | 7,926,000
Benefits paid, net of plan participants’ contributions |  |  |  |  | (4,479,000)
Fair value of plan assets at end of period |  |  |  |  | 83,926,000
Funded status |  |  |  |  | (176,077,000)
Postemployment employee (liability) |  |  |  |  |
Current | (800,000) |  | (800,000) |  | (800,000)
Noncurrent | (127,837,000) |  | (127,837,000) |  | (175,277,000)
Total | (128,637,000) |  | (128,637,000) |  | (176,077,000)
Affordable Care Act, Cadillac Tax Repeal | OPEB |  |  |  |  |
Change in Benefit Obligation |  |  |  |  |
Benefit obligation at beginning of period |  |  | 260,003,000 |  |
Service cost |  |  | 1,354,000 |  |
Interest cost |  |  | 2,004,000 |  |
Plan participants’ contributions |  |  | 58,000 |  |
Actuarial loss (gain) |  |  | (44,017,000) |  |
Benefits paid, net of retiree subsidies received |  |  | (2,342,000) |  |
Benefit obligation at end of period | 217,060,000 |  | 217,060,000 |  | 260,003,000
Change in plan assets |  |  |  |  |
Fair value of plan assets at beginning of period |  |  | 83,925,000 |  |
Actual return on plan assets |  |  | 4,676,000 |  |
Employer contributions |  |  | 2,106,000 |  |
Benefits paid, net of plan participants’ contributions |  |  | (2,284,000) |  |
Fair value of plan assets at end of period | 88,423,000 |  | 88,423,000 |  | $ 83,925,000
Funded status | $ (128,637,000) |  | $ (128,637,000) |  |

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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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