SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Postretirement Benefits (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2017
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Normal retirement benefit as percentage of final average compensation (in hundredths)	1.20%
Number of consecutive years considered in final average compensation	5 years
Number of prior years considered in final average compensation	10 years
Maximum service period (years)	25 years
Normal retirement additional benefit	1.00%
Maximum additional service years	10 years
Percentage of average monthly compensation in excess of covered compensation for each year of credited service	0.65%
Maximum period of credited service (years)	35 years
Covered compensation period (years)	35 years
Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution cost percentage	50.00%
Minimum age for eligibility before amendment	55 years
Contribution percentage before amendment	40.00%
Retiree premium percentage	100.00%
Cost of coverage contribution	50.00%
Premium contribution percentage after amendment	50.00%
Minimum | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Retiree age group range	55 years
Maximum | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Retiree age group range	65 years
Defined Contribution Profit Sharing, Savings and Investment Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined contribution plan, employer non-discretionary contribution amount	3.00%
Contribution cost percentage	50.00%
Defined contribution plan, employer matching contribution, percent of employees' gross pay	6.00%
Defined contribution plan, employer contribution, vesting period (years)		5 years