The number of plans sponsored by entity.

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- Definition

Defined Contribution Plan, Maximum Employer Contribution, Percent

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Amount of cost for defined contribution plan.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 70
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480794/715-70-50-1

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

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Percentage employer matches of the employee's percentage contribution matched.

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Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

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EMPLOYEE BENEFIT PLANS - Defined Contribution Plans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) plan	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
401(k) Plan
Defined Contribution Plan Disclosure [Line Items]
Number of plans	1
Employee contribution, maximum proportion of eligible earnings (as a percent)	75.00%
Employer contributions made \| $	$ 4.9	$ 3.9	$ 3.3
409(a) Plan \| Hired After May 1, 2021
Defined Contribution Plan Disclosure [Line Items]
Maximum employer contribution (as a percent)	10.00%
Non-bargaining unit employees, excluding Colowyo Coal employees \| 401(k) Plan \| Hired Prior To May 1, 2021
Defined Contribution Plan Disclosure [Line Items]
Employer contribution, maximum proportion of eligible earnings (as a percent)	1.00%
Non-bargaining unit employees, excluding Colowyo Coal employees \| 401(k) Plan \| Hired After May 1, 2021
Defined Contribution Plan Disclosure [Line Items]
Maximum employer contribution (as a percent)	10.00%
Non-bargaining unit employees, Colowyo Coal employees \| Maximum
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution, maximum proportion of employee contributions (as a percent)	5.00%	5.00%	5.00%
Non-bargaining unit employees, Colowyo Coal employees \| 401(k) Plan
Defined Contribution Plan Disclosure [Line Items]
Number of plans	1
Non-bargaining unit employees, Colowyo Coal employees \| 401(k) Plan \| Hired Prior To May 1, 2021
Defined Contribution Plan Disclosure [Line Items]
Employer contribution, maximum proportion of eligible earnings (as a percent)	7.00%
Maximum employer contribution (as a percent)	10.00%
Non-bargaining unit employees, Colowyo Coal employees \| 401(k) Plan \| Maximum \| Hired Prior To May 1, 2021
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution, maximum proportion of employee contributions (as a percent)	5.00%