Additional discretionary contributions to defined contribution pension plans.

No definition available.

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- Definition

Defined Benefit Plan Amortization Period.

+ References

No definition available.

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- Definition

Number of Defined Contribution Plans

+ References

No definition available.

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- Definition

Paid discretionary contributions to defined contribution pension plans, company match.

+ References

No definition available.

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- Definition

Percentage of actuarial gains and losses amortized.

+ References

No definition available.

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- Definition

Percentage of company's qualified pension plans on a GAAP accounting basis.

+ References

No definition available.

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- Definition

For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Accumulated Benefit Obligation

-URI http://asc.fasb.org/extlink&oid=6503844

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920

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- Definition

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Expected Long-Term Rate of Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512013

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(3)

-URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920

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- Definition

Amount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position.

+ References

No definition available.

+ Details

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- Definition

Amount included in accumulated other comprehensive income (AOCI) related to prior service cost (credit) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position.

+ References

No definition available.

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- Definition

Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position.

+ References

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- Definition

The amount of decrease that relates to an irrevocable action that relieves the employer (or the plan) of primary responsibility for a benefit obligation and eliminates significant risks related to the obligation and the assets used to effect the settlement. Examples of transactions that constitute a settlement include (a) making lump-sum cash payments to plan participants in exchange for their rights to receive specified benefits and (b) purchasing nonparticipating annuity contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Section 15

-Paragraph 6

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=7577095&loc=d3e8001-114927

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)(10)

-URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920

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- Definition

Target allocation of investments of this type to total plan assets presented on a weighted average basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(i)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)(i)

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- Definition

The amount of cash or cash equivalents contributed during the reporting period by the entity to fund its pension plans and its non-pension postretirement benefit plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

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- Definition

The amount of pension benefit costs recognized during the period for (1) defined benefit plans and (2) defined contribution plans. For defined benefit plans, pension expense includes the following components: service cost, interest cost, expected return on plan assets, gain (loss) on plan assets, prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. For defined contribution plans, the pension expense generally equals the firm's contribution to employees' accounts (if the firm contributes) during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942

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-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

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-Subparagraph (h)

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Pension and Savings Plans (Details Textuals) (USD $)	12 Months Ended
Pension and Savings Plans (Details Textuals) (USD $)	Dec. 31, 2015	Dec. 31, 2014 years	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Estimated prior service cost that will be amortized from accumulated other comprehensive loss		$ (11,400,000)	$ 1,400,000	$ (20,100,000)
Settlement Charge		28,300,000us-gaap_PensionExpense	51,800,000us-gaap_PensionExpense	32,400,000us-gaap_PensionExpense
Defined Contribution Plans [Abstract]
Number of 401(k) plans		3btu_NumberOfDefinedContributionPlans
Paid discretionary contributions to defined contribution pension plans, company match		44,700,000	46,400,000	51,100,000
Additional discretionary contributions to defined contribution pension plans, performance feature		18,300,000	16,500,000	22,500,000
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Settlements, Benefit Obligation		41,700,000
Actuarial gains and losses amortization corridor, percentage		5.00%
Period of amortization (in years)		5
Estimated net actuarial loss that will be amortized from accumulated other comprehensive loss	39,600,000
Estimated prior service cost that will be amortized from accumulated other comprehensive loss	1,000,000
Expected rate of return on plan assets	6.25%	6.85%	7.75%	8.00%
Accumulated benefit obligation for pension plans		1,002,500,000	947,300,000
Minimum funded percentage defined by the Pension Protection Act of 2006		80.00%
Pension and Other Postretirement Benefit Contributions		3,900,000
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	4,800,000
Settlement Charge		8,700,000	0	0
Other Pension Plan, Postretirement or Supplemental Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Contributions		$ 1,700,000
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for assets of Master Trust		35.00%	35.00%
Fixed Income Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for assets of Master Trust		65.00%	65.00%
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for assets of Master Trust		4.00%	4.00%