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EMPLOYEE BENEFIT PLANS - Narrative (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Employee benefit plans |  |  |
Maximum participants' contribution as a percentage of pre-tax compensation | 20.00% |  |
Employer's matching contribution for each dollar contributed by the employee | $ 0.75 |  |
Maximum employer contribution as a percentage of base salary | 6.00% |  |
Company contributions | $ 6,500,000 |  | $ 6,000,000.0
Annual rate of increase in the per capita cost | 5.50% |  |
Health care cost trend rate assumed, year three | 5.00% |  |
Health care cost trend rate, benefit obligation | 3.80% |  |
Period to reach ultimate trend rate, benefit obligation | 40 years |  |
Health care cost trend rate, net periodic benefit cost | 4.00% |  |
Period to reach ultimate trend rate, net periodic benefit cost | 53 years |  |
Fixed Income |  |  |
Employee benefit plans |  |  |
Assumed percentage of portfolio investment | 40.00% |  |
Assumed long-term inflation rate | 2.10% |  |
Long-term rate of return on plan assets | 3.44% |  |
Equity Securities |  |  |
Employee benefit plans |  |  |
Assumed percentage of portfolio investment | 60.00% |  |
Assumed long-term inflation rate | 2.10% |  |
Non U.S. Equities |  |  |
Employee benefit plans |  |  |
Long-term rate of return on plan assets | 9.17% |  |
Short-term cash investments |  |  |
Employee benefit plans |  |  |
Long-term rate of return on plan assets | 2.57% |  |
Minimum | Fixed Income |  |  |
Employee benefit plans |  |  |
Maturity period | 1 year |  |
Maximum | Fixed Income |  |  |
Employee benefit plans |  |  |
Maturity period | 5 years |  |
S&P Index |  |  |
Employee benefit plans |  |  |
Performance benchmark of special index | 35.00% |  |
Russell 2000 Index |  |  |
Employee benefit plans |  |  |
Performance benchmark of special index | 15.00% |  |
MSCI EAFE Index |  |  |
Employee benefit plans |  |  |
Performance benchmark of special index | 10.00% |  |
Bloomberg Barclays U.S. Aggregate Bond Index |  |  |
Employee benefit plans |  |  |
Performance benchmark of special index | 40.00% |  |
Pension Benefits |  |  |
Employee benefit plans |  |  |
Accumulated benefit obligations | $ 622,000,000.0 | $ 615,500,000 |
Fair value of pension plan assets | 716,794,000 | 573,575,000 | 469,774,000
Net periodic benefit cost | $ 45,684,000 | $ 34,374,000 | 41,552,000
Long-term rate of return on plan assets | 6.50% | 6.25% |
Average return for last five years | 11.90% |  |
Estimated annual contributions in next fiscal year | $ 23,500,000 |  |
Pension Benefits | Fixed Income | Level 1 | Money Market Fund Investments |  |  |
Employee benefit plans |  |  |
Fair value of pension plan assets | $ 9,600,000 | $ 8,900,000 |
Pension Benefits | Total Domestic Equity: |  |  |
Employee benefit plans |  |  |
Long-term rate of return on plan assets | 8.18% |  |
SERP |  |  |
Employee benefit plans |  |  |
Accumulated benefit obligations | $ 78,000,000.0 | 71,800,000 |
Other Benefits |  |  |
Employee benefit plans |  |  |
Fair value of pension plan assets | $ 147,057,000 | 128,554,000 | 102,625,000
Threshold retirement age for participation in plan on payment of a premium | 58 years |  |
Life insurance benefit | $ 10,000 |  |
Net periodic benefit cost | 5,211,000 | $ 7,880,000 | $ 8,832,000
Regulatory asset related to underfunded postretirement benefit expense | $ 1,500,000 |  |
Long-term rate of return on plan assets | 6.00% | 5.50% |
Average return for last ten years | 9.40% |  |
Estimated annual contributions in next fiscal year | $ 2,200,000 |  |
Other Benefits | Fixed Income | Level 1 | Money Market Fund Investments |  |  |
Employee benefit plans |  |  |
Fair value of pension plan assets | $ 3,100,000 | $ 4,500,000 |
Other Benefits | Total Domestic Equity: |  |  |
Employee benefit plans |  |  |
Long-term rate of return on plan assets | 7.86% |  |

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