EMPLOYEE BENEFIT PLANS - Defined Benefit Plan (Details) $ in Millions		1 Months Ended	12 Months Ended
	Jan. 01, 2013	May 31, 2013 USD ($)	Dec. 31, 2023 USD ($) employee	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jan. 01, 2022	Jan. 01, 2021
Defined Benefit Plan Disclosure [Line Items]
Number of employees			1,120
RS Plan | Multiemployer Plans, Pension
Defined Benefit Plan Disclosure [Line Items]
Employer contributions to plan | $			$ 31.6	$ 25.2	$ 26.7
Funded amount (as a percent)						80.00%	80.00%
RS Plan Contribution Prepayment Option | Multiemployer Plans, Pension
Defined Benefit Plan Disclosure [Line Items]
Prepayment amount, multiple of annual required contribution	2.5
Reduction in the annual contribution (as a percent)	25.00%
Reduced annual contribution period			15 years
Contribution prepayment | $		$ 71.2
Amortization period of prepayment	13 years
Collective bargaining agreements and their bargaining unit employees
Defined Benefit Plan Disclosure [Line Items]
Number of employees			215
Maximum | Non-bargaining unit employees, Colowyo Coal employees
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution, maximum proportion of employee contributions (as a percent)			5.00%	5.00%	5.00%
Colowyo Coal
Defined Benefit Plan Disclosure [Line Items]
Number of employees			192