EMPLOYEE BENEFIT PLANS (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Savings Plan
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	$ 5,600,000
Pension and SERP
Expected future benefit payments
2020	15,851,000
2021	17,528,000
2022	19,328,000
2023	21,165,000
2024	22,932,000
2025-2029	143,248,000
Total payments	240,052,000
Pension plans
Employee benefit plans
Accumulated benefit obligations	615,500,000	492,400,000
Fair value of pension plan assets	573,575,000	469,774,000	460,878,000
Net periodic benefit cost	34,374,000	41,552,000	35,900,000
Expected future benefit payments
2020	13,765,000
2021	15,332,000
2022	16,992,000
2023	18,749,000
2024	20,517,000
2025-2029	130,341,000
Total payments	215,696,000
SERP
Employee benefit plans
Accumulated benefit obligations	71,800,000	56,800,000
Expected future benefit payments
2020	2,086,000
2021	2,196,000
2022	2,336,000
2023	2,416,000
2024	2,415,000
2025-2029	12,907,000
Total payments	24,356,000
Other benefits
Employee benefit plans
Fair value of pension plan assets	$ 128,554,000	102,625,000	100,563,000
Threshold retirement age for participation in plan on payment of a premium	58 years
Life insurance benefit	$ 10,000
Net periodic benefit cost	7,880,000	$ 8,832,000	$ 8,451,000
Regulatory asset related to underfunded postretirement benefit expense	2,100,000
Expected Benefit Payments Net of Medicare Part D Subsidy
2020	3,305,000
2021	3,645,000
2022	4,011,000
2023	4,417,000
2024	4,864,000
2025-2029	29,403,000
Total payments	49,645,000
Effect of Medicare Part D Subsidy on Expected Benefit Payments
2020	(267,000)
2021	(301,000)
2022	(336,000)
2023	(371,000)
2024	(404,000)
2025-2029	(2,656,000)
Total payments	(4,335,000)
Expected future benefit payments
2020	3,038,000
2021	3,344,000
2022	3,675,000
2023	4,046,000
2024	4,460,000
2025-2029	26,747,000
Total payments	$ 45,310,000