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%