EMPLOYEE BENEFIT PLANS - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2023
Employee benefit plans
Maximum participants' contribution as a percentage of pre-tax compensation	20.00%
Employer matching contribution and percent of match	75.00%
Company contributions	$ 7,400,000	$ 6,500,000	$ 6,500,000
Recorded value of noncurrent investments	$ 63,900,000	$ 57,300,000
Other postretirement plan	Other Postretirement PlanThe Company provides substantially all active, permanent employees with medical, dental, and vision benefits through a self-insured plan. Employees retiring at or after age 58, along with their spouses and dependents, continue participation in the plan by payment of a premium. Plan assets are invested in mutual funds, short-term money market instruments and commercial paper based upon a similar asset mix to the pension plan. Retired employees are also provided with a $10,000 dollar life insurance benefit.The Company records the costs of postretirement benefits other than pensions (PBOP) during the employees' years of active service. Postretirement benefit expense recorded in 2021, 2020, and 2019, was $0.2 million, $5.2 million, and $7.9 million, respectively. The remaining net periodic benefit cost was $0.8 million at December 31, 2021, and is being recovered through future customer rates and is recorded as a regulatory asset.
Annual rate of increase in the per capita cost	5.40%	5.40%
Health care cost trend rate, benefit obligation	3.90%
Period to reach ultimate trend rate, benefit obligation	38 years
Health care cost trend rate assumed, year three	5.00%
Health care cost trend rate, net periodic benefit cost	3.80%
Forecast
Employee benefit plans
Ultimate health care cost trend rate				5.20%
Domestic Equity: Large Cap Stocks
Employee benefit plans
Index percentage	35.00%
Domestic Equity: Small/Mid Cap Stocks
Employee benefit plans
Index percentage	15.00%
Non U.S. Equities
Employee benefit plans
Index percentage	10.00%
Long-term rate of return on plan assets	8.73%
Fixed Income Securities
Employee benefit plans
Index percentage	40.00%
Fixed Income
Employee benefit plans
Assumed percentage of portfolio investment	40.00%
Assumed long-term inflation rate	2.20%
Long-term rate of return on plan assets	3.34%
Equity Securities
Employee benefit plans
Assumed percentage of portfolio investment	60.00%
Assumed long-term inflation rate	2.20%
Short-term cash investments
Employee benefit plans
Long-term rate of return on plan assets	2.35%
Minimum | Fixed Income
Employee benefit plans
Maturity period	1 year
Maximum
Employee benefit plans
Employer matching contribution and percent of match	6.00%
Maximum | Fixed Income
Employee benefit plans
Maturity period	5 years
Pension Benefits
Employee benefit plans
Accumulated benefit obligations	$ 650,200,000	$ 622,000,000
Fair value of pension plan assets	810,469,000	716,794,000	573,575,000
Net periodic benefit cost	27,253,000	$ 45,684,000	34,374,000
Defined benefit plan, effect of change in yield curve on net periodic benefit cost	12,200,000
Defined benefit plan, effect of change in method on net periodic benefit cost	$ 6,200,000
Long-term rate of return on plan assets	6.34%	6.50%
Estimated annual contributions in next fiscal year	$ 16,100,000
Pension Benefits | Fixed Income | Level 1 | Money Market Fund Investments
Employee benefit plans
Fair value of pension plan assets	$ 11,000,000	$ 9,600,000
Pension Benefits | Total Domestic Equity:
Employee benefit plans
Assumed long-term inflation rate	7.89%
SERP
Employee benefit plans
Accumulated benefit obligations	$ 80,500,000	78,000,000
Other Benefits
Employee benefit plans
Fair value of pension plan assets	169,436,000	147,057,000	128,554,000
Life insurance benefit	10,000
Net periodic benefit cost	227,000	$ 5,211,000	$ 7,880,000
Net periodic benefit cost	800,000
Defined benefit plan, effect of change in yield curve on net periodic benefit cost	1,200,000
Defined benefit plan, effect of change in method on net periodic benefit cost	$ 1,100,000
Long-term rate of return on plan assets	5.88%	6.00%
Estimated annual contributions in next fiscal year	$ 700,000
Other Benefits | Fixed Income | Level 1 | Money Market Fund Investments
Employee benefit plans
Fair value of pension plan assets	$ 3,000,000	$ 3,100,000
Other Benefits | Total Domestic Equity:
Employee benefit plans
Long-term rate of return on plan assets	7.65%