Number of persons employed by the Entity

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- Definition

Under the prepayment option, the amortization period of the prepayment contribution, which was recorded as a deferred charge, made to the plan.

+ References

No definition available.

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- Definition

Under the prepayment option, the amount of the prepayment contribution made to the plan.

+ References

No definition available.

+ Details

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- Definition

Under the prepayment option, the percentage reduction in the annual contribution to the plan.

+ References

No definition available.

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- Definition

Under the prepayment option, the period of the percentage reduction in the annual contribution to the plan.

+ References

No definition available.

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- Definition

Under the prepayment option, the approximate number of times the annual required contribution to the plan used to determine the prepayment amount.

+ References

No definition available.

+ Details

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- Definition

Amount of accumulated benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920

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- References

No definition available.

+ Details

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- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Amount of contributions made to multiemployer plan by employer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 9
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-Publisher FASB
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-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

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- Definition

Indicate whether the multiemployer plan is less than 65 percent funded, between 65 and less than 80 percent funded, or at least 80 percent funded. The acceptable values are: (1) "less than 65 percent", (2) "between 65 and less than 80 percent", (3) "at least 80 percent", and (4) "NA". A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Indicates (true false) whether employer contribution represents more than five percent of total contributions to multiemployer plan for most recent plan year available.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (e)(2)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

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EMPLOYEE BENEFIT PLANS - Defined Benefit and Defined Contribution Plans (Details) $ in Millions				1 Months Ended	12 Months Ended
	Jan. 01, 2019	Jan. 01, 2018	Jan. 01, 2013	May 31, 2013 USD ($)	Dec. 31, 2019 USD ($) employee	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
DEFINED BENEFIT PLAN
Number of employees \| employee					1,467
Executive benefit restoration obligation included in accumulated postretirement benefit and postemployment obligations					$ 0.7
DEFINED CONTRIBUTION PLAN
Employee contribution, maximum percentage of eligible earnings					100.00%
Contributions made					$ 3.5	$ 4.6	$ 3.2
Collective bargaining agreements and their bargaining unit employees
DEFINED BENEFIT PLAN
Number of employees \| employee					280
DEFINED CONTRIBUTION PLAN
Contributions made					$ 0.0
Non-bargaining unit employees, Colowyo Coal employees
DEFINED CONTRIBUTION PLAN
Employer contribution, maximum percentage of eligible earnings					7.00%
Non-bargaining unit employees, excluding Colowyo Coal employees
DEFINED CONTRIBUTION PLAN
Employer contribution, maximum percentage of eligible earnings					1.00%
Maximum \| Non-bargaining unit employees, Colowyo Coal employees
DEFINED CONTRIBUTION PLAN
Employer matching contribution, maximum percentage of employee contributions					5.00%
Defined benefit pension plan \| Collective bargaining agreements and their bargaining unit employees
DEFINED BENEFIT PLAN
Number of employees \| employee					280
RS Plan \| Defined benefit pension plan
DEFINED BENEFIT PLAN
Our contributions represent more than (true) or less than (false) 5 percent of total contributions					false	false	false
Contributions made each year					$ 25.8	$ 27.8	$ 26.7
Plan funded status	At least 80 percent	At least 80 percent
RS Plan Contribution Prepayment Option \| Defined benefit pension plan
DEFINED BENEFIT PLAN
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
Colowyo Coal
DEFINED BENEFIT PLAN
Number of employees \| employee					225
New Horizon Mine \| Collective bargaining agreements and their bargaining unit employees
DEFINED CONTRIBUTION PLAN
Employee contribution, maximum percentage of eligible earnings					1.00%