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EMPLOYEE BENEFIT PLANS (Plan summary) (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 USD ($) plan person | Mar. 31, 2017 USD ($) person | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Pension and other postretirement benefit plans |  |  |  |  |
Noncash charge to pension settlement expense |  |  | $ 4,000 |  |
Nonunion Defined Benefit Pension Plan |  |  |  |  |
Pension and other postretirement benefit plans |  |  |  |  |
Nonparticipating annuity contract purchase price | $ 14,000 | $ 7,600 | 14,000 |  | $ 7,600
Number of plan participants for which vested pension benefits were settled | person | 120 | 50 |  |  |
Transfer of remaining obligation to PGBC | $ 1,500 |  | 1,500 |  |
Number of plan participants for which benefit obligation was transferred to PBGC. | plan | 30 |  |  |  |
Employer contributions | $ 7,700 |  | 7,710 | $ 5,500 |
Noncash charge to pension settlement expense |  |  | 4,000 |  |
Postretirement Health Benefit Plan |  |  |  |  |
Pension and other postretirement benefit plans |  |  |  |  |
Employer contributions |  |  | $ 848 | $ 769 |
Unrecognized prior service credit, amortization period |  |  | 9 years |  |

X
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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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